+1. His medical tattoo can do more than filter extra O2 from the water.
The chlorine gas permeated the water, a thick, choking miasma. It was an accident; some quirk of fate-Kaldur knew what would happen if he inhaled the pool-water, knew what would happen if he breathed in the burning death-
Under no circumstances should you ever attempt to breathe in a "swimming pool". The water, while filtered and oxygenated, is also chlorinated, and will kill you in a very painful way. Never, ever, no matter what, should you ever breathe pool-water.
When he was younger, new to being Aqualad, one of the surface exchange students (for the sciences) had died in a pool accident- and he had helped create a public service announcement, as a part of being Aquaman's (his king's) protégé.
But that was three years ago, and right now he's tangled up in some kind of net thing- it's sharp and buzzing with electricity, but with a little mental twist his tattooed foci are absorbing the electrical charge- but the person tangled up with him- they can't breathe, and he at least can give them a little longer before-
Kaldur presses his mouth to Roy's, lips locking- his tongue shoots out, and forces Roy's mouth open. He was told, repeatedly, that this technique was beyond his capabilities- he would only hurt himself if he tried- but his oldest surface friend, I don't have very many, is about to die and they can be awkward together later on the surface where neither of them are about to die of asphyxiation. (They're both going to die and they know it- but I can't give up, I can't-)
Kaldur begins to breathe for them both. He tenses sharply, waiting for the sharp sudden rush of burning inside his chest- but it doesn't come. He feels a soft tingling where his tonsils used to be- and the sweet, blessed rush of oxygen to his blood. With another little mental twist his magic is shunting unneeded oxygen through his secondary trachea, and out his mouth, directly to Roy.
(Due to a tiny quirk of his genetics, Kaldur'ahm actually uses a slightly smaller percentage of oxygenated water than the average Atlantean. This means that the Shared Breath Technique is slightly less dangerous for him to use.)
Later, when they are both on dry land, (Kaldur's in a kiddie-pool, but the water is barely an inch deep) Roy will sling his arm around Kaldur's shoulders. There will be a slight (huge) amount of awkwardness in their future relationship- Kaldur saved Roy's life; Kaldur kissed Roy, and he maybe-sorta-kinda liked it. Roy is grateful to Kaldur; Roy kinda-sorta-maybe wants Kaldur to kiss him again.
They're sixteen; give it time.
Okay, so- sorry it took so long to get this last part out; this chapter refused to be written.
Head-cannon that Atlanteans have two tracheas- one for the gills, one for the lungs- add some magic and they can manipulate which one gets used when.