Saw this one on a night dive in Barbados and also saw one of these on a night dive in Saint Croix. The locals in Saint Croix nicked named it the beast but no one new what it was. It is very shy and starts to contract when you approach it.
It's a type of sea cucumber - common name is "medusa worm" Next time touch it - they are sticky!
Cheers
James
Disclaimer: no holothuria were harmed during the making of this post of the touching of said holothuria...
James is right. Euapta lappa is the name you'll find in guide books for the Caribbean. There are other similar looking species in the region.