Video here.
It’s clearly a whatdafuckdebeast.
A piece of whale gut moving in the current?
Baby leviathan.
Large plastic sheet, like a tarp.
Moving from IMHO to General Questions.
IF it is a sea creature, and not just something caught in a current, I think it may be related to sea jellies (although it also bears some resemblance to an octopus or nudibrach [sea slug]). Beginning at 4:15, the video shows some “organs”, what possibly is an orifice and some kind of organ for stinging or delivering reproductive cells.
I’m not convinced that it is alive, but that’s my best guess.
And** Barcus **has it nailed.
The pulsating movement of the thing on the video corresponds with the camera movement - I think a lot of the apparent animation of this object/creature is caused by the manoeuvring thrusters of whatever vehicle the camera is mounted on.