Looks like it has a triangle shaped projection, one bigger, one smaller, sticking off the top, and then some other sort of 'head' breaking the water in front of those.
If it's a critter, my suspicion would be that it's not of terrestrial origin, IE a windowfaller. Either that, or it's normal habitat has been so disturbed by undersea volcanic activity that it's been driven toward land. It's totally possible that it's some sort of garbage I suppose, but it seems to surface and then sink.
Could be a fun question for the C's just to see what they say.