Pista agassizi would be my best guess. Found in fouling community under a dock. Not sure about its burrow, since someone else found it. Didn't get shots where you can count the branchiae.
ID'd to Amphinomidae by Dot Norris. From there the only tricky part of the Light's key is the "caruncle" which on this worm is the little wavy thing extending dorsally from the head back two segments. Branchiae present on all segments.
I found this worm pumping for ghost shrimp in sand, so it's possible it was living comensally in a ghost shrimp burrow.
These are super common in the sand, but I've never dug one up before. I think I broke it, and didn't get decent enough photos to get to species, I suspect.
Large, broad, flat head, too big for the rest of the body. Bottom of body flat.
Dead, floating on the surface. Bright purple color, about 6mm.