[Squirrelloid]

Attacking Underwater nations:
The UW-Land conversion is really really poor in the base game. Try Burnsaber's UWGIM (UnderWater Gameplay Improvement Mod) or CBM 1.9x (think 1.92 is the latest, almost entirely incorporates UWGIM) for a better cross-sea level experience. FWIW, the experience is at least as bad, if not worse, in the other direction. UW powers tend to have a really hard time invading land and staying there.

That said, the following is how you should go about doing it:
(1) 1500 troops is mostly chaffe. Thugs and SCs can more than pull their weight, especially against chaff armies, and especially ones who naturally are amphibious.
(2) Some troop types are naturally amphibious. For most nations this is mostly summons. In addition to specifically amphibious troops, all undead are poor amphibians, and since you're not fielding them for their stellar stats, the stat hit isn't all that relevant.
(3) Astral magic is probably the best offensive school for underwater combat, followed by air (most lightning spells work) and water (for School of Sharks/Shark Attack). Most evocations simply don't work underwater.

How do i make an effective Super Combatant (SC)?
(1) Encumbrance is key. Thugs or SCs which build up fatigue are going to die in protracted battles. You absolutely must be fatigue-neutral (reinvigoration >= encumbrance). Remember buffing can cause a lot of fatigue. Remember if you Quicken Self you'll attack twice per turn *and* accumulate 2x the fatigue.
(2) Defenses: You absolutely must have pretty good MR (22 is a minimum for SCs, thugs can get by with less, but <18 is asking for it). You're also going to want to stack a high-parry shield, high protection, and some other defensive measures (like luck, elemental immunities, awe, etc...).
(3) Fear is amazing vs. chaff and pretty decent against anything that isn't immune. THE MANUAL IS WRONG ABOUT HOW FEAR WORKS. Fear *decreases* morale of enemies in an AoE around the fearful unit, so when you attack and deal damage to units, and thus provoke morale checks, the enemy is more likely to break. Since it does this *without limit*, a fearful opponent who doesn't die *will* rout any enemy not immune to fear. (Morale 30, 50, 99 are special numbers and immune to fear).

(4) Thugs are meant to be budget province takers, not army killers like SCs (although they can supplement armies well). Throw a good weapon and shield on some chasses and you can pretty reliably do this. Some chasses may require slightly more gem investment. Encumbrance can be less important (because the scope of engagements is more limited), although many thug chasses are undead and thus Enc 0. Moreso than SCs, a lot of thugs rely on being sacred and blessable to perform well as a thug because it reduces gear needs. (Which isn't to say there are no sacred SCs, but rather that SCs are a more gem-intensive investment and thus you aren't demanding the same sort of gem-efficiency out of them, so needing a bless to perform isn't as much of an issue).

That should help get you started with Thug and SC construction.

(SC pretenders come in two flavors: Early game and Mid game. An Early game SC pretender needs very little or (preferably) no gear to clear provinces, and can expand from turn 1. It will become less useful as time goes on because these chasses tend to not have full slots, and thus not be fully gearable. A mid-game SC pretender is typically also a bless chassis, and taken asleep or imprisoned. Its meant to run with full gear and kill enemy armies. This can last into endgame against the AI or bad players, but even midgame use against good players is risky and endgame use is suicide in MP - not something you'd want to risk your god on).