The best way to find out is to Google dino spawns for whatever map island or rag but mostly in various waterways. I haven't seen them in anything that was just a lake or pond. Also saw something saying they are tied to fish spawns so if there are only fish then kill alot and come back later and give more otters a chance to spawn.
Yup slaughter all fish and return later is your best bet if you don't find them, sometimes they spawn in while your killing the fish off if your lucky. Also look on land around water, they do wonder up and in to the bushes