I know and admit that I blocked most of the water intake grill for heads with antifoul paint, thinking 'Oh I must remember to clear that before launch'.
To (belatedly) close the loop on this...
I decided to careen the ship in Lymington - this was a new experience for me and it went rather well. The grid was lovely, with a nice clean concrete base and excellent piling.
As suspected there was a MASS of densely packed hairy weed stuck in the inlet seacock. It took some poking from above and below to clear it.
Glad I didn't attempt this with the boat in the water. On the balance of drying out vs water pouring in, drying out was a much more pleasant experience and I got to give her a wipe down too.