Answer
How deep should a marina slip be?
Add at least 1 ft of cushion to your boat's draft at the lowest tide. Deep-draft sailboats and yachts often need 8+ ft of controlling depth.
Depth by boat type
Outboards and pontoons: 3 ft minimum. Cruisers and small sailboats: 5 ft. Trawlers and 40+ ft cruisers: 6–7 ft. Yachts and full-keel sailboats: 8+ ft.
Tide considerations
Marinas report controlling depth at MLW. In areas with 6+ ft tide swings, the difference between high and low tide can be the difference between safe and grounded.
What 'controlling depth' actually means
Controlling depth is the shallowest point on the path from open water to your slip — including the entrance, fairway, and slip itself.