noob: how to get troops underwater

[Darkwind]

I assume you mean Niefelheim? They have water magic, so I'd suggest researching Construction and forging some items that give Water Breathing (for example the Sea King's goblet at Construction 6, which gives Water Breathing 100, or enough for 100 troops). There's also a global enchantment, Thetis' Blessing, that allows all land troops to enter the water.

Unfortunately, water nations have received no such love and aquatic troops can't really ever enter land, from what I know.