[Ironhawk]

The primary way for a land nation to go underwater is via summoning. You could also hunt around for a shambler or ichythid land province and then build an amphibious army from there.

Another good way is to build a fairly tough underwater-capable SC. Then use it to punch in to a weaker underwater province and establish a "beachead" (maybe I should say seahead?). Then pour all your resources into building aquatic troops from there and constantly reinforce it with summons and amphibious troops until you are ready to expand outward from there.