since i'm mostly playing ulm (have to, i live there

), i have a problem when facing underwater nations (espacially r'lyeh). i can repel their attacks, throw them back into the sea if they conquer one of my provinces.
but i want to take the battle to them. ulm (at least with my setup) isn't that good at magic. i usually go with earth and fire magic, which gives some good items, and fire magic helps in land battles. underwater of course it's nearly useless.
conquering an amphibian inhabited province seems the best way so far. i'm curious whether there are better ways. would empowering work? give a commander or spellcasting mercenary water magic?