Eels are tough possible early-game enemies xD
There are some even nastier water creatures, too, overall tough because fighting these things in the water puts you at a disadvantage, and they won't generally surface enough at range to kill with ranged attacks... I had to get by one to reach a chest on the other side of water once, and best I could do was magically pacify it for a bit while I slipped by :P (in some cases it can be effective to use a mercenary/ally to coax them out at the water's edge and use ranged attacks while they attack your friend...)