[Agema]

My suggestion for underwater expansion are elite units if you're going to take land troops.

Basic infantry have about 10 all stats. With the underwater penalty of -3 Att/Def, you're looking at troops hitting about half the time and being hit 50%+ more often - think of it as taking a bunch of standard militia into combat on land.

This means you need to take huge amounts of troops to get combat equivalence, and that will cost you in gems making the waterbreathing items. Furthermore, you'll be doing a lot of swiping and missing, which added to an underwater encumbrance penalty will mean you also tire very quickly, exacerbating your troubles.

If you take elites, you'll be fighting with approximate parity, as they'll still have 9 or so Att/Def after the penalty. Even better, elite cavalry, as you can get them heavily armoured, significantly increasing survivability, whilst being mounted means their armour will fatigue them much more slowly. They'll often have two attacks (hoof, sword, lance on first turn) as well, meaning they'll still give you a fair advantage against average aquatic troops despite the penalties.