What is this strange mushroom-like ocean critter?

i am at st. simons island, georgia right now, and today i swam out to this sandbar about 100 yards off the beach. there were these strange organisms in the shallow water sticking out of the sand. they resembled a mushroom. they were purple and had about a two inch long "stem". when i picked it up and put it on the sand it sorta looked like it was breathing. they had these bumpy spore kind of things on the top. a little kid walking by told us it was a sea cucumber, but unless it was a very strange kind, i don't think it was. sea cucumbers are actually in the shape of a cucumber and this looked like a mushroom. what is this strange thing?!?!