I can talk about your concerns first hand. Last year my trailer was tied up with other boats on it for most of the season and my bowrider sat in the creek where I keep it for probably 4 months straight....
here's how it looked when I pulled her out
The worst was the port side which had sun exposure continuously. The starboard side (shaded by the dock) was cleaned using industrial toilet bowl cleaner and then a good scrub with starbrite hull cleaner followed by a wash and wax. the port side and rear were cleaned with the same industrial toilet bowl cleaner, CLR and a pressure washer... The result was the strings of algae and the "blackness" were removed but there was still a cement like residue stuck to the hull. This spring I had to use CLR and a scotchbrite pad to completely scrub the hull down. It was 4 hours of very hard labour! Now I am polishing with my buffer to restore the shine!
My recommendation ---if you are going to let your boat sit in a slip, Have it bottom painted to the water line. They make bottom paint in all different colours (even white) . You will still have to scrub just above the water line but atleast your boat is protected from osmosis.
