[thejeff]

You'd be better off forging Amulets of Breathing and sending Hochmeisters in. Or wait for higher construction levels.

There's not really a lot you can do with trinkets and solo weak units.
2 weapons and Black Steel Plate. Underwater so a shield isn't a priority and none of the trinket shields have special properties.
Be prepared to go through quite a few of them. Icthyids are pretty tough, for indys. You'd be better off going after weak triton provinces, if there are any.

Edit: You've got mermen, not just captains. Build as many mermen as you can (at least 2-3x the icthyids) and swarm them.