I've taken a shedload of Storm/Frost Atronachs down at dark anchors, they seem to randomly spawn now and again, i have also had them appear when random rifts appear in the air, unsure about water based creatures but im wondering if crocodiles and such from the swamp areas count?

nerids are so far the only critters i have found that seem to count for water based sadly they seem few and far between and there respawns are quite long up to and over 20 mins any and all daedric /netches can be found aplenty in cold harbor lvl 50 content I highly suggest you perk and enhance fighters skill silver bolts then morph to silver shards(silver leash is pointless and costs double stamina to drag things close to you ) before attempting to solo lvl 50 plus daedra this skill is a vital skill as it is effective aginst 3/4 the content pre vr ranks however in vr content its effectiveness drastically drops off due to the shere volume of health mobs have sadly one the server epic failures of vr is your skills donot scale to content .....

Water Based creatures are "Ice Wraiths" those flying fish bone look alike spirits found in Eastmarch and Lion's Den Dungeon (Snowy Part).

Since you are ebonheart pact I suggest going into the crows wood portal in stonefalls and farming the wraiths in that beginners area.

I am in the EP and cannot find a good place to farm Storm Atronachs and Frost Atronachs for this achievement. Dolmens drop them sometimes but it is quite annoying to run from dolmen to dolmen.

I'm not up to Cadwell's Gold yet, just Silver. I could do with some info on where to farm Atronachs. Any ideas?

EP btw, so no Dominion territory yet.