Um, I'd avoid arctic maps and go with temperate or tropical, and savage ocean if possible (give yourself a chance at sea serpents). The oceans have the single biggest ammount of biodiversity in the ENTIRE game outside of the tropical environments-you simply cannot match it with underground features. And as far as I know, no fish have building destroyer. No fish, by the way, can be tamed without modding-and there have been issues with them airdrowning when taming was turned on. I almost WISH that my fish were nice normal ones which stayed in the ocean-my walls are looking like boatmurdered's but with FISH instead of elephants.
Undead fish do have benfits-they can come onto land and get caught in traps there. They also have a *big* drawback-they can come onto land and kill your dwarves. For instance, I have had skeletal whales pay me a nice visit. Four dwarves dead before I realized what had happened. I even thought, how nice, that pet werewolf is going to help the dwar-Urist McForgothisname has been struck down. Oh, and it's impossible to tame the things. On the bright side, your exhibits will last for eternity. Just plop zombie fish in tanks of formaldehyde.
BTW, dragons *are* inate swimmers, correct?