Hi there -
Only my second reply ever -
I have a 1800MX 2006, living in South Wales, UK.
We have a fiberglass deck, and snap in carpets, after every outing we hose and wash the floor area down, and tip the boat upwards on the trailer to let gravity help.
I know exactly what you are talking about, some of the water goes through under the fiberglass engine cover, but the majority sticks under the seats as the gap is not big enough for the water / dirt to flow underneath. (engine in midddle with fiber glass / plastic cover, and two single seats either side. (3.0L mercruiser EST engine)
Eventually if you leave it up in the air for 10 mins it will drain, then just use a cloth or a wet and dry hoover to suck up the dirt. As someone else mentioned - probably not a good idea for the dirt to be washed behind anyway, especially as it will probably just rest under the fuel tank or the batter / power steering pack.
I am having issues with my boat at the moment - no spark! I will post into another forum -
All the best,