Will my 20ft pontoon boat be ok in a wet slip at the lake all winter?

I live in Texas, and the slip my boat is in is on Lake Tawakoni. I plan on changing my lower unit oil and grease, and more than likely getting a tune up done. If it will be fine in the water over the winter should I trim the motor all the way up or down? I have a 50 hp Nissan outboard motor.