There are some troops that are amazing under water. Stuff people usually don't use there though
You don't need many of the following along your commander, so perfect for a land-lubber commander wanting to get their feet wet.
Minotaurs, elephants, chariots, iron pigs and what ever tramplers you can think of. Who cares if they can hit or not, they just trample anything in their way. There are no arrows under water to annoy them
Then a couple more specialised options (available for only a few nations)
Hydras. Oh em geee. In an on-going MP game as LA Pythium (with CBM1.6 though, which makes it much easier to get under the waves) the hydras are just great. Many under water thingies have poisonous attacks, but AFAICR nobody has poison resistance. Hydras chew through anything under the waves, and again, no arrows to annoy them. Hydras don't have to hit anything, they just have to not die (and the hydras are excellent at not-dieing).
Monster toads. Ok, these are tramplers *and* they have poison cloud and it is easy to give them a small Nature bless. Does it get any better than that under water where there are no arrows?