[PrinzMegaherz]

Or you can use fto speed up the battle.

Anyway, back to the main topic:

First choice of troops to conquer land provinces are your amphibian troops. There should be an icon looking like a pale lizard in their details. Second, you might want to research construction as you might want to forge items that allow your water only commanders to lead your armies out of the sea (but I'm not sure how many water only commanders atlantis has).

Anyway, have a look at the shore provinces you could attack and look for a province that has some useful troops in it, like archers or cavalry. this might be the perfect position for your first shorebound castle, as it gives you the possibility to recruit troops you the atlantians themselves do not have.
And rejoice, for some reason, many water evocations don't work under water, if you have done some research in evocation, your mages usefullness will definitly increase on land