You know you want some nudes. Well, Nudibranchs

Depends. From my recollection, some of these are pretty venomous (if not all). They tend to 'steal' venom from their environment like those poison frogs. Incorporating venom from their food into them. That being said, I'm not sure the level of lethality to humans. I do know that a good number are non-lethal to humans but the danger from touching them mostly comes from the fact that they're delicate bois.

Edit: From some basic googling, it seems that none are known to be lethal to humans but there is one, the first one in the image "The Blue Dragon", can give you a pretty nasty poke because it eats the Man o' War. Dangerous but not necessarily lethal.