It depends on the water and the temperature.
I remember when I was a kid we left our boat in greenwood lake(NY/NJ) which was relatively cold freshwater for the entire season with no problems and very little, if any, discoloration or growth.
In other locations, the tannic acid in the water may leave an ugly brown slime very quickly, and in other areas algae may become a problem.
In warm saltwater like where I live, I don't even like leaving it in for the weekend due to barnacle growth.
Generally speaking, in cool freshwater extended periods shouldn't be a problem, in saltwater you need to be more careful.