Pulled out of water on my 89 bard with Boastful Bellow after clearing a frog on the NW corner of the lake. Pulled him between a tree and the little fountain near the wall. Simple enough kill with 89/3kAA Beast/Bard box and tank/healer merc. Had no problems getting him out of the water or tagging him from just inside the lake. Didn't try to attack from shore because I have never seen that work in this game on underwater mobs. Hit max I saw was 5.9k, confirmed 12min respawn.
Was flagging my 91 SK for the Gardner task. Jumped in the water with my 85 bard box. Was not able to do any damage to him underwater. No spells or songs landed and "immune" to my melee. I use """ because I think its a bug. Was able to kill him on land. The only way I could agro him from underwater, was with bash.
it seems he is on a 12min respawn timer, engage him when repop, else he wanders around for some minutes to get back into the water. Under water there are nearly no adds but strangely, no spell lands, but the mercs can kill him melee. But if you get over water you get wasp as adds. Easiest: stay under water, attack and then go back and let the mercs and pets do there work while a healer merc heals the tank merc... they stay under water relatively safe....