A/N: As I'm not a sea-faring person myself, nor particularly marine inclined, forgive me if I used some terms incorrectly in this story. As always, JK owns the Potterverse.
Barnacles.
Why did it always have to be barnacles?
Yes, Harry had swam through the freezing Black Lake for a competition he really wanted no part of, and that was an experience he certainly didn't want to repeat any time soon, but there weren't that many barnacles hanging around in the lake (other than right in the merfolk village, and Harry was a bit too busy focusing on rescuing some people than staring at underwater architecture). But this was just ridiculous.
"Luna dear, could you come down here for a minute?" shouted Harry. "We need to have a talk about something."
"Just a minute!" Luna shouted back. Harry waited as his girlfriend climbed down from her perch atop the ship, and came down to meet Harry near the stern of the vessel.
"All right, Harry, what's going to disturb my pirate time?" Luna asked, eyepatch and captain's hat still firmly in place. Harry pointed to the mass of barnacles clinging to the bottom of the boat. A quick flip of the patch let Luna put both eyes on the problem, and another flip put the patch back in place when she had taken a look. "It's just a bunch of sea-life, Harry, all the good old pirate ships had them. I'll clean them off when they become bothersome, but until then, they're staying right where they are."
Harry sighed. "Luna, you remember that we don't own this ship, right?" Luna nodded. "And that the only reason you have pirate time at all is because Viktor pulled some strings at Durmstrang to get this boat, on the promise that we returned it in the same condition we borrowed it, right?" Luna nodded. "And if we don't keep the barnacles off the ship, they'll eventually take it over, and you'll have to explain to the headmaster of Durmstrang why you weren't able to hold up your end of the bargain and return a ship in less-than-expected condition?" Luna kept her head down, but hugged Harry tightly.
"I'm sorry, Harry," she whispered, "it's just been so long since I was able to play Pirate Ship like I used to with my parents that I forgot how much work went into actually using a real ship. It's so much easier when it's just a conjuration, and barnacle buildup isn't a problem." She held him tight again, tears slowly trickling down her cheeks.
Harry kissed his girlfriend on top of her head. "It's all right, Luna. I tell you what, how about we go out and buy a boat of our own? Maybe not a grand old pirate ship, but something we can have friends on, sail around the world in if we like, and yes, maybe even play Pirates on once in a while. Would you like that?"
Luna's head perked up so fast her captain's hat flew backwards and she tried very hard to snog the life out of her boyfriend. Said boyfriend gently took off her eyepatch and stuck it in his pocket (not an easy feat by far while having arms full of excited blonde witch) before letting her regain her footing. "I'll take that as a yes." He stooped to pick up her hat, and walked hand in hand with his girlfriend back up towards their beachside house.
Before they got out of sight of the ship, Harry stopped and turned Luna to face him. "There is one last thing we have to do before we go boat shopping, though," said Harry.
"And that is?"
"Someone," and here, Harry poked Luna lightly in her stomach, "has to clean off Durmstrang's ship before we send it home, and it's not going to be me."
"Barnacles."
