My boat is kept outdoors. I've learned that the boat will start to look nasty being kept this way after a few weeks. The most obvious culprits are dust and little particles of vegetation, especially these little round pieces that I think are coming from the palm trees. Once the plant matter stays on the boat for a while it discolors the gel coat and requires a really good scrubbing. How much structural damage the gel coat sustains from the sun I don't have the experience to know. But I think with regular washing and waxing before it gets too dirty should keep it in good condition. I don't think water should make a difference as long as it can drain and evaporate.
So I've been looking at boat covers and they're expensive. I'm also not sure of their durability, and suspect there's a lot of heat built up under them as they're not really made to allow air to circulate. So I got an idea to use this material and I want to know if anyone has used it before for a boat cover. For my patio I installed about a year ago these Coolaroo sails. They're basically very durable shade cloth, and relatively expensive because they're chic nowadays. They'll keep the rain out some but are water permeable, they're made of loosely woven synthetic fibers, almost like very thin fishing line.
The main thing they do is provide shade while still allowing air circulation (these pics are not my house).
Once I had the coolaroo sails for a while I wanted to install them in other parts of the house. But because they're expensive I tried to find something comparable that's just labeled shade cloth. I ordered some material online that looked similar but the construction was completely different and looked inferior so I returned it. I was at Harbor freight recently and noticed what they call a mesh tarp. Surprisingly it was much more similar to the coolaroo fabric than the other stuff I bought, and looked durable. Not at all like the blue or brown tarp which I know disintegrates in the Florida sun after a few months.
This one is 12' x 19', perfect size for my boat. I'm thinking that it will definitely stop the dust and vegetation, provide some sun protection and at the same time allow air circulation so things don't get too hot underneath. In heavy rain some water will make it into the boat but it will drain, and the rest will evaporate quickly because of the air circulation. The only thing I don't like is the color but for $50 vs. a $400 cover you can't complain. I think it's worth a shot, but maybe someone has experience with this material for a boat cover and can give me a heads up.