The real enemy of the of boat is the sun and the UV. The water only in that case if it cannot be dried out easily and starts to create mildew.
Under this, the best cover is protect against the UV, after raining it gets dry quickly and above all, easy to put on.
For this, the lycra cover from UVcovers is the best solution. very light, stretchy, fits perfectly to the boat, very easy to apply and if gets wet, dries out awesome fast. Above all it comes with nice colors.
Any type of catamarans, dinghies or big boats, the choice is this cover.
Fro more info check out the Facebook page