Not only have I seen them, I've killed them... My seal-tribe hunter happened to establish a camp on the northern coast and a large pod of seals were found inhabiting the area nearby. They seem to like small rocky island chains near the shore.
they are tough little buggers to kill... they come ashore sometimes, so i endeavored to hide in a ditch near the shoreline, wait for one to come past me, then sprint out and try to cut off its escape back to the water. They're faster than you'd think, and can take a ton of damage. By the time I finally got one, he had like 5 javelins sticking out of him and a broken flipper, and i still had to stand there for a while and hammer on him with a club before he finally gave up the ghost.
I gave up hunting them after that - too much effort for too little gain. Maybe traps would be better? Perhaps you could use fish as bait? Not sure if it works or not.