Didn't see it!!!
Its pretty ironic as i was actually trying to avoid a clump of floating seaweed at the time, this stuff was actually under the water , weighted down by a lead weight and a stone or two.
theres no protection from this at all ,or so it appears, apart from being vigiliant.
I think the speed limit means your in contact with the seaweed for longer and perhaps at higher speed it wouldn't get sucked in so easily. I was on the mouth of the Arun, which is a fast flowing estuary, at high tide, which may also explain why there was submerged seaweed.
anyway, i'm determined to get back out tomorrow, weather premitting but may launch from the public ramp which is closer to the sea!