The Eidolon of water is Ultros, and it takes some more effort than merely naming them to actually summon them, promise.

[ All right, there was a look in Clarus's eyes that eased something in Regis, so he's focused on what is going on, again. He took the gem from Salieri, too, smiling with his thanks.

Besides, he knows this - he is a summoner, and he's been doing his homework. ]