This is very unlikely to be seen, and here one of aoe2 original dev explains why:
Basically, water units doesn’t have the complexity of the land unit, and generally you just spam them, alternating between fire and galleys, with some demos here and there. They had a lot of generic unsued ships for all civs, and they deliberaly didn’t used for this reason, and because it would make water balance a pain in the ***.
Even for water UU, how many times do you actually see them, the longboats are the exception, turtles received a ton of buff and still you don’t seem them very often, lastly caravels were introduced just as a “longboat killers”, and they are mostly useless outside of that.
If they are able to add some new water UU with eventually new civs, that is fine, they may be able to squeeze in a couple more of them (units that will most likely be very situational) but hard no to a water UU for all civs or even for just some old civs.
Now this, this I would like. New civs, with new bonuses and UT, some of that focused on the water could be a good idea, if you avoid making new “italians” (meaning a civ that is good only on water and useless on land).
I made a design for a civ some time ago based on the venetians, which would perfectly fit this this hypotetical DLC:
I even designed a water UU for them, and belive me, it’s super difficoult to do so.