Sirius' eyes are half shut to block out the sun this far up in the water, close enough to the surface that the waves and the thrashing, spinning school of large silver fish he's chasing are making so much noise, so densely packed, that he can't hear Remus scream for him to stop.
That he doesn't see the shadow of the boat.
Or the net.
Sirius hits a wall of trapped fish, their heavy bodies slamming into his from all angles and he's too slow to get out before the massive, thick netting is pulling but and Sirius is shunted into the bottom along with too many fish in tight quarters as the entire thing hauls up. He curls up with his arms over his head, fish buffeting him.
Remus pulls at the net and screeches, Sirius singing right back and it bounces off each other, echoing. The fish thrash harder, cut bloody where they press against Sirius' tail. The net is thick and won't cut, not fast enough, Sirius is half buried in all the fish, he can't get out enough to climb out the top of the not quite closed net.
Sirius is dragged up, out of the water and there's a blast of sound, of heaviness, the fish crushing him. He gasps, coughs out water, spluttering and wheezes in air. He doesn't go up to the surface, he's a Siren, not a Mer.
Remus hangs off the net as it rises but the pressure is too great and he drops with a large splash. Sirius can't reach him, caught almost in the centre with his tail stretching all the way down, fin shoved painfully against the net. He feels the net sway, moving sideways now.
He can't see anything, it's too bright to open his eyes all the way and there's too many fish, but he can hear Remus signing. He's swayed through the air some more and then dumped in a sudden rush and splashes into water again, slamming against the bodies of fish.
He thrashes, knocking the heavy fish away and bursts up, only to slam against metal grating over the top. He claws at it, shrieks, ducks his head and rams his shoulder up into it. There's movement above and Sirius' head snaps up, squinting through the sunlight.
Humans in bright orange and wearing thick disks over their ears, they crowd a bit closer and point. One pulls off the discs and flinches, quickly shoving them back on as the siren song Remus is still calling out reaches them.
"They can't hear, they're blocking it!" Sirius yells.
Remus' song slows, bursts out again and then dies away.
"It's fine!" Sirius calls. "It's fine, I'll get out! Protect the nest, the ship is too close!"
"…et hel…ames and…"
Sirius can barely hear Remus over the loud clanking of the human ship and the battering of waves against the hull. The sound moves different out of the water, it's almost overwhelming.
Sirius claws around the edge of the grate, needs to keep batting fish away. The water rolls like the ship, there's no stability and every time he reaches his arm out to grab the grating they feel weaker, so heavy above the surface.
The grate won't move, and while Sirius bends a few bars, it's made sturdy. Sirius swims down but the water is almost all red from how many fish he's accidentally gutted. He squeezes his eyes shut to give them a break from the sun but still sees starbursts of light behind his eyelids.
He doesn't know how long he spends but the sun doesn't get easier on his eyes and he starts to feel sick from the movement of the ship.
The humans crowd closer, Sirius swipes for one but the grate restricts him and they jump back quick enough to dodge. They take things out of pouches and bags, point them at Sirius. More humans show up, some move away.
Sirius sings but mostly all of them wear the things over their ears, those who don't stay inside the ship and stare down from behind glass. Sirius spins in circles, slams his tail against the side of the container.
It's fine, it's fine. Remus has definitely made it back to the nest. He's protecting their Mers, Sirius just needs to focus on getting out.
A flash of glittering brown arcing across the bright sunlight overhead.
James slams a few humans into the ground and slides across the wet deck, short claws scrambling for purchase before he grabs a pole, whips his tail around and hurling two humans aside where they ragdoll and bang against the railings.
Sirius and James lock eyes for a split second and then the ship tilts with the waves and James is pushing off, slipping away and through the railing, disappearing just as suddenly as he arrived.
The humans are screaming, rushing towards their downed companions, some leaning over the edge to see where James went. Sirius remembers an old conversation, where Harry said that James was the one who dealt with human ships. Sirius believes him now.
James comes diving back onto the deck, slams into the grate above Sirius with a clang and they're reaching through the crosshatching, clutching at each other's arms. James shrieks, slaps his tail against the deck and bares his teeth at the humans. The humans have all backed away, grabbed new things now, pointing them too.
James takes a moment to look at the grate, sees the seams that Sirius couldn't from his vantage point. "This," he says, disentangles a hand from Sirius to grab a big box-like this. "This, can you break this?"
Sirius moves over and reaches through, grips it and it starts to bow under his strength. Something fast darts by and James screams in pain, whipping around and half curling up.
"Go!" Sirius cries. "James, go, I can get out."
James pushes off the grate, rising onto his hands. He grunts at the humans, slaps his tail and uses the same grunting noise.
The humans start grunting back like - like they're talking?
"James, just go," Sirius hisses and with one last violent yank, the box is torn off.
James drags himself off and in the meantime, Sirius ducks down into the water so he can burst back up, ramming open the gate that tips up and clangs onto the deck on the other side. Sirius claws his way out, dragging his massive tail that's so heavy now that he's out of the water, his arms are shaking just keeping himself propped up, he has no idea how James can even move properly up here.
James grunts again and they're both gone with the next ship-rolling wave.
They fall for much longer than Sirius will ever be okay with and hit the water with a large splash. "How the fuck did you jump that high?" Sirius blurts out, grabbing James and frantically swimming down, half dragging the slow Mer because of James' tail fin.
Or not his fin, because red plums trail behind them coming from James' tail, midway up and on the left. Remus and Harry meet them under the boat and Remus takes James' other arm and they swim fast.
Harry darts ahead, gets the medicine ready, which is basically chewed up types of seaweed. They get James settled in the sand and Harry hesitates.
"It's a hole," Harry says. "Does it work if it's not a cut? How do I…?"
"How deep is it?" James groans, writhing a bit but holding himself together.
"I – I don't…" Harry looks at Sirius and Remus.
Remus leans in and gently waves a hand, using the current to wash away as much blood as possible. "This part is still bleached, there are new scales underneath, the injury couldn't go deep."
"Just put it on," James sighs and grabs Sirius hand, gritting his teeth and Harry slathers the medicine over the wound as gently as he can. As soon as Harry leans away James lets out a groan and slumps.
"The blood will call others," Remus says worriedly. "Would the humans stay?"
"They usually leave after I kill a few," James murmurs, having gone limp. "They might come back, the ones at the reef did."
"How did you make them stop?" Sirius asks, still keeping eyes overhead for anything incoming.
"Kept killing them until they decided it wasn't worth it," James admits. "You can…the big thing on the back of the ship, the thing that spins, get the human net from the oval trench nearby, wait until the ship is far enough away and get it tangled. They'll call for help. Wait and kill them when they come down to the water."
"We have to guard you, at least two of us," Remus says firmly. "And we can't go solo after the ship. Leave the ship, we can move again, find a new trench."
"Sirius is more than enough to stay with dad," Harry states. "I can see in the sunlight, I'll go with you after the ship."
"It's not worth it," Remus snaps. "Sirius was almost taken and James got a hole put in him!"
"And we should make sure they don't come back to finish the job!" Harry yells back.
Remus grits his teeth, looks to James and Sirius. "...We're faster than the ship. We stay for the first few days, then we'll go."
Harry dives back down to James, checks on his tail, falls over his chest with a shaky sigh.
"I'm okay, it's not like I was using the tail for much anyway," James jokes. It falls a bit flat.
They feast for weeks.
.
A/N: Later, they find out James was raised in captivity and then saved by the Potters and slowly released back into the wild. It's how he can speak to humans and he was taught the jump too, plus a few more tricks.
A human research boat comes by to the regular place and James swims all the way out to meet them, tells them about regular Mer life in the wild in return for food. They also patch up his tail for him. Harry meets his human grandparents and it's very cute.
Harry also brings back an injured Siren named Tom…