Mark, Hi.
We used to have a Colvic 30ft centre cockpit boat which on our first cruise we took from Brundall to York and then to Lincoln. We were moored in the Brayford and needed to get under the 'Glory Hole' in order to get back home in time for my next flight.
"It won't go, Dad!" said Giles........... he was correct, by about 4 inches....
We just didn't have the time to go all the way back down the Trent and Humber, plus the journey from Spurn Head to Gt Yarmouth... so I disconnected the cooling water input pipe and opened the sea-cock. I remember I had to flood the bilge to nearly half way up the engine to get us that extra headroom! We then poled under the Glory Hole and I then started the engine with the pipe still disconnected and pretty soon sucked the bilge almost dry.
Yes, it is possible but the pitfalls associated with doing it are various and obvious to most, particularly the sequence of opening and shutting the seacock.... As Mark said, whether I'd want someone else to do it with my boat is a completely different matter! :naughty: