Poop-Off All Marine Bird Poop Remover
It works. Came back to boat, last year, to find that the cormorants had decided my spreaders were the "in place" for dining. Poop-off removed their leavings from the sailcover and dodger beautifully.
Doesn't get any better. Actually it does, new boat is Freedom which has no spreaders.
Can be found in pet supply stores and Amazon.com