[Endoperez]

I think that if going underwater needs either pretender with Water or Water Cult, who not use a nation that can use Water blessing together with Water Cult? Vanir of course, and to lesser extent Tuatha, White/Black Centaurs and Serpent Dancers benefit from Water Blessing, and are reasonable units to try to take on underwater. Especially with some help from the pretender.

I am not skilled enough to answer to Chazar's second post, but in some cases the option of going underwater might be worth the points. It might be worth it for Vanheim, whose Vanir could retreat to seas if an oversea-attack failed... But I don't even know if that is possible