Is this maritime plant a species of zostera?
It certainly looks like a Zostera species.  The ones that appear in the UK are Zostera angustifolia, Zostera noltii, and Zostera marina.  Its hard to tell exactly which species this one is without size information and knowing the conditions of the bay.  Z. marina has the broadest leaves at around 5 - 10 mm, whilst Z. angustifolia has leaves just 2 - 3 mm broad.  Is 40m around the mid-water mark?  If so I would hazard a guess this is Z. angustifolia.  Z. marina tends to stick to areas permanently covered by the sea (sub-littoral) whilst Z. noltii likes much more sheltered conditions than the middle of the bay, tending to stick to the upper shore.

Hope that helps!