[elmokki]

In the few singleplayer games I've tried I found that Lady of Rivers or whatever she is called seemed to be the most useful way to get underwater - rather than taking a wyrm and good scales I rather had her and some magic.

Jomon's troops' biggest problem is having no shield - and underwater you don't need a shield. The poor amphibian penalty isn't too bad since your units have above average stats anyway.

Sure, as soon as you are under water you want proper underwater troops, but before that it'll do.

I did try a cyclops with underwater breathing item, but it seems far more risky.