Actual, the answer is as flat as possible. Sure dont want to have an easy path to take the vessel down to the water in the morning, then after a hard day of sailing have to drag 500 lbs uphill . ADA requirements want about a max 5% slope, but you can get by with a greater slope for less distances, but have landings in between. Just cause water is usually down the beach, you are going to have it downhill somewhat. I would try and shot for around 1-2% for your slope. That is roughly what you see on a cross slope in a road (from the centerline to the curb/gutter). But like I said, the flatter the better.

Jonathan Grant
Mesa, AZ
Boatless