So wrapping it all up, if you have 300 something point slot in your army and you want to dish out some damage, you can go with 6 eels of whatever flavour or 3 sharks and nothing is wrong with that. Some say, also a turtle is nice (the batallion with turtle, shark and eels sounds like a good idea).
But I dont know about the eidolons. You cant compare the damage output easily as they are full of abilities and spells. So maybe someone with a lot of experience can tell us when and why to take which eidolon. I know some was explained here and there on this forum (tricks with steed of tide etc.), but till now I dont get the whole picture.
I realy would like to build and paint one over the holidays, but only if he is useable. I even dont know which one to take.