The "hairy" non-ramified brown algae
RSEV_328.
Chorda sp. (Chorda filum) is not as "hairy", generally longer and is hollow, so tends to stay more vertical (erect) underwater due to its increased buoyancy.
depth of 1.2m (ajusted to chart datum)

A little something to brighten @thomaseverest 's and @pliffgrieff 's and @susanhewitt 's day - thank you for looking at all those old bivalve observations recently. Not sure what this is, my ID is a guess, but the shell was about the size of a nickel and it was 'swimming' vigorously but stuck in floating giant kelp when I first spotted it. Looks vaguely like fileclams from the tropics!? Also a little further north than most other observations for this species on iNat (but range is to Monterey, according to https://www.msi-norco.ucsb.edu/field-guide/index/index/guide/artreef?species=580). Video: https://drive.google.com/file/d/1K52ZsgJTSuIXlXeOJp4vne3zAV_osx1B/view?usp=sharing

Common Name: Northern Cerianthid

Location Found: Found while diving Pump House Beach off the East Point of Nahant, MA. Found in approximately 23ft of water. Not attached to hard substrate, but buried in the sand/substrate.

Habitat Types: Found in sandy/muddy bottoms from 20' to 1000'+ from the Arctic to Cape Cod.

Physical Description: This species has two whorls of tentacles that are placed near the mouth and to the outside of the mouth. The inside whorl is shorter than the outside whorl. Colors can change on the tentacles and the central column of the species. They can grow to a foot and a half tall and up to 1.5 inches wide, with tentacles extending much further.

Fun Fact: These anemones don't have a pedal disk as they don't attach to hard substrate, but rather bury themselves in sand/silt!

Reference: Martinez, "Marine Life of the North Atlantic"