[Nikolai]

It is hard. Against AI, magical super combatants are best. AI cannot fight them. Summon Wraith Lord, give him ring of water breating, give him equipment, cast Soul Vortex, and kill dozens of troops.

Against a player, good Luck! I am watching a friend play a game in which he is killing player under water. He (my friend) has heavy infantry. A manual of water breathing lets them walk under water. Some wizard/priests lead, and cast Leagions of Steel, Strenght of Giants, Body Etherial, Luck on the heavy infantry, and then cast battle spells. It works, but by the end, most infantry is unconscious from tired. He said he is lucky he is fighting land nation as well. Water nation would have native water mages, and he would be killed.

All in all, fighting under water is same as fighting over water, but harder to get people breathing. Otherwise, an army with many super fighters, troop shielding and tons of mages works ;-)