Species of the genus dendrobates, one of my favorites while living in Costa Rica, are commonly called poison dart frogs. This one is a green and black poison dart frog (Dendrobates auratus). We used to catch the strawberry poison dart frogs while living in La Selva but I've only seen a couple of the green/black variety and that was closer to the coast.
This friendly looking snake is a southern pacific rattlesnake (Crotalus oreganus helleri), native to southern California.
This is a banded Sea Krait (Laticauda colubrina), probably one of the most venomous snakes in the world. There is no treatment for a bite from a sea snake; however, they typically are not very aggressive and rarely bite people. I believe my dad had a bit of run in with one of these (or a close relative) while diving in Indonesia. His was unusually aggressive and I believe my dad had to exit the area rather quickly. Was this also when the incident with the trigger fish occurred?
This is a rather fuzzy picture of a Lionfish (Pterois volitans), also a highly venomous creature. They may be beautiful but I would not recommend crossing paths with this guy. He is aptly named.