Look for the bubbles on the surface. Use some form of water-breathing, such as Totem of the Otter etc, it'ds a long dive. Follow the bubbles downward.
The entrance to the cave is at ( 50, -99, 394 ). And I agree with Sigrdifa you should take a Totem before going down there. Good luck and have fun!
I usually prefer not finishing this questline so I can keep Griz. I'd rather have the bear hat than the crappy mount. Unless they've added another polar bear hat at low levels somewhere and I haven't kept up.
If you're doing the Frostfang Sea timeline, Thirin Veliumdelver on the Cragged Spine gives you Totem of the Iceclad Manta as part of his quest Below the Frozen Waves. It gives water breathing (but only in Frostfang Sea) and lasts 5 minutes, which is enough time to get down to the cave and kill a few orcs along the way (don't start using it till you swim out above where the entrance is, before you submerge). It has 5 charges, and his dialogue says you can get more from him if you need them. (I've never gone back for more myself.)
