It was about a fisherman (ithink) who caught a seal who when it took off it's skin became a beautiful woman. He took her as his wife. She escaped near the end of the movie when she found her skin

I think she was sunbathing as a woman and he stole her skin so she stayed with him.

When she turned back into a seal she went to her home that was like a mountain or volcano that inside had a beautiful late with many seals/selkies

It was on TV back in the late 70's early 80's
I live in the US

Is it possible you have two movies mixed in your memory? I was searching with your description, and found the crater lake type scene, but its from The White Seal.. at 5min, you see the seals entering the crater, and an ending scene from above showing all the seals around the lake...
http://www.youtube.com/watch?v=z0blTVuPgJA&feature=related

No. I know it was one movie. I remember the movie clearly just not the name