That's definitely a skate, probably Raja binoculata (big skate). The resemblance to a guitarfish, which has a somewhat similar triangular head/midsection and a well-developed tail, is caused by the claspers on this mature male which makes the tail seem thicker. The claspers are those long extensions on either side of the tail. The claspers start off small in immature individuals and then grow faster proportional to the rest of the body as males approach maturity. I used to see them pretty often when diving over sandy or muddy bottoms, but their numbers appear to be declining. I've caught skates on conventional gear before and it was a tussle. I imagine that it was real battle on a flyrod.