It could have been a Manta Ray, though I have no clue; I didn't even see the thing, but from the wake the sucker left in the silt, it was BIG.
If you know any traveled surfers, mention the word "Witches Cove" to them or ask about it. The outfit I use in Costa Rica has 2-3 guys from San Diego and LA (Long Beach, I believe) that are seasonal to the area. Great place to surf as the last time I was there, it was me, my friend, our guide, 3 miles of EMPTY beach and good waves.
The Pacific side of Costa Rica is a surfer's paradise, as you don't have to fight crowds most of the time, except in Tamarindo Beach.
