I have owned several boats, trust me, no boat should ever be left uncovered, instead, get a boat cover. A lot of different pages in this site deal with what to park your boat under. Here we will suggest how to cover your boat.
Each of the companies below will sell RV covers in many types and sizes. This page can be used as a quick reference for online cover shopping.
Notice how this boat is strapped to the trailer, this is important. Anytime you trailer a covered boat the cover must be properly secured. I learned this the hard way once, cost me a nice cover. The wind tore the crud out of it. I never pulled the boat covered again.
Below is a classic boat cover by Stellex, one of my favorites and a great deal.