I know that if they bleed out underwater, you can find their corpse floating not too far away.
I killed a seal by hitting it with a broadhead arrow from a treeline while it was sunning itself on a little rocky outcrop offshore. It dove into the water, and I went back to camp to get my raft. When I got back, I searched the waters and found its corpse floating maybe a hundred meters or so to the north. That's the only seal I've ever killed though.