Re: Bridge Clearance_How little is too little....
Yes the clearance is MHW or Mean High Water in tidal areas. They often mark the heights at the base of the bridge.
If you're sure about your height, 4 ft clearance is not too little. I'd make sure I knew when tides were higher than normal and watch any big wake machines.
There's a low bridge and a power cable in Cape May canal that I cleared with inches helping a friend move his boat. We waited for low tide. Inches were nerve-racking..