Yeah, brings to mind (see web site San Francisco bay sea serpent) the "two brothers watching a giant sea serpent while parked drinking thier morning coffe next to the sea wall on San Francisco bay-

and the sea serpents was chasing seals and on of the serpents swam up tp the sea wall and looked at the brothers and hisssed at them.

The main sea serpent of the lot was Ginormus!! hugh!! and the seals were going nuts!!

A crocodile/alligator doesn't strike me as very likely. I believe that salt water croc's mostly find their home on the other side of the world, and I've never heard of a salt water species of alligator.

Also, the shape of a croc/alli's head is pretty distinctive. If it had popped out of the water and hissed at them, I'm pretty sure they would have been able to correctly identify it.

It seems like pretty odd behavior too. It was threatening them, and from the description seems like it could have killed them if it had wanted. If it was defending a nest, part of thinks that might have have attacked right out.

Perhaps it was a juvenille and was just playing with her as it would one of its siblings. That would be my best guess anyway.

Conjecture aside, this is a very exciting story! I doubt anything will come of it, but its really interesting to see new sea creatures appear to this many people at once. Wish I could have been there!