When I was off camping, there was a colony of seals off the beach nearby
Now while it was close it wasn't really close enough to get a good look at them but there were a fair few dead pups washed up on the shore, as grim as it may sound, inspecting carcasses is a very good way to learn about anatomy
Here is a sketch of seal, alive and well whose anatomy is derived from such studies
I still don't think I got the flippers quite right, especially the back right one they look more like feet than that

I did get to see one live seal quite close after swimming a river and climbing over many rocks, that was pretty cool