My daughter got married last November, and her husband's family held the rehearsal dinner at Mitchell's. They had a separate room just the right size which set up beautifully. We had three choices of entrees all of which were fabulous along with appetizers and dessert options. It could not have been nicer. I highly recommend it.
Watercolor has a couple of possible venues which would be very nice. Baithouse and Boathouse on Western Lake. October tends to be a busy month, IMO the best month of the year, so early res. should e made. Good you are looking now.
Check when the various homeowner's weekends are - they are usually in October and that will make a big difference in availablilty of restaurant space and lodging.
The Beach Club Restaurant is a great location for a rehearsal dinner! We have a private room for up to 50 people that overlooks the gulf PLUS a ton of delicious menu items to choose from. If you're interested, our Wedding & Special Events Coordinator Christine Walker can give you more details. (850.267.1240 or christine@santarosaclub.com)