Confidence: It's likely to be this, but I can't be certain.
Notes: the lengths of seaweed that you have, you might reasonably expect to see some small round air bladders of this weed. Sargassum muticum is not recorded in your area yet (not saying you haven't got it - it's really invasive), you don't really want it either.
Sea Oak - has compressed fronds as in pics.Branching in one plane. Pretty likely to be this.