|No, not one of these. Although ...|
First and foremost, we're going to replace their legs with sea turtle-like flippers. The fins are clawed, so they can keep their claw attacks, but they also gain a swim speed. Question - swim speed equal to land speed, or faster. With their flippers, their land speed should now be about half what it used to be. Do they keep their wings? Not sure.
Obviously, the beasts can breath in water, but it might be worth making them amphibious. Depends, I suppose, on whether you want them following adventures out of the sea. Personally, I'd remove their ability to breath air entirely - leave the surface to the traditional dragons and the water to the aquatic dragons. Besides, trying to outrun an enraged aquatic dragon to the surface could make for a neat challenge in a game.
Since they spend their time underwater, you might want to replace a dragon's chance of speech with a chance for telepathy.
As for the different breeds:
Black - Black dragons are already semi-aquatic, so it might make sense to leave them alone. If I were going to do a purely aquatic version, I might make them bottom dwellers who mostly lurk in the very deep, dark oceans, where their scale color gives them good camouflage. Since many deep sea fish are rather weird-looking, that might be fun to carry over to the aquatic black dragons - a glowing form of bait, maybe one that detects as magical to draw in greedy adventurers. As for their acid breath, I'd maybe make it an acidic slime that coats their body or a cloud of acid they belch from their mouths.
Blue - The lightning-breathing blue dragons seem to mesh nicely with the idea of electric eels. I'd replace the lightning breath with a shocking ability, and make it extend to some radius - maybe 10 feet, maybe more, maybe tied to the dragon's size. I think giving them an eel-like face and body would also be cool. Maybe they make their lairs among the coral reefs.
Green - Imagine aquatic green dragons lurking among the kelp beds. Since they're known for being poisonous, what if they have poisonous spines like a puffer fish or maybe they're based on sting rays, with similar body shapes and stream-lined heads.
Red - Red dragons are all about fire, which makes their presence in the water tough to deal with. They could make the water super-heated and boiling, though, so that might work. I might give them a more mottled appearance - maybe more purple than red. Perhaps they dwell around submarine volcanoes, and thus are immune to poison and fire, and perhaps they have a weird appearance like those deep sea aquatic black dragons. Better yet, let's give them hammer heads because, well, because it would look cool. In fact, basing red dragons on sharks would be a pretty good idea, given their similar outlooks on life. If they don't dwell around volcanoes, I'd put their lairs in sunken treasure ships.
White - The first thing that pops into my head when thinking about aquatic white dragons is them dwelling in nomadic pods in the cold, arctic seas. In fact, an orca theme might work well with aquatic dragons, with maybe some white seal fur and walrus tusks. White dragons aren't known for being terribly smart, so we can make these fellows more dumb predator than mastermind. Rather than a frost breath, they could breath a cloud of chemicals that flash freezes the water and everything in it.
I'd love to hear your ideas about "aquatizing" the chromatic dragons in the comments.