It makes sense to close any seacock when you're not on board the boat, i.e., it's unlikely to sink from an open seacock if the seacock's are shut. Having said that, does anyone on here know of any boat that has actually sunk due to a dodgy seacock ?

Of course, the difficult thing to do is to remember to open your raw water intake(s) before frying your engine, so many people just don't bother.

I'd open the bog seacock when on board, and shut it before you step off the boat, but it's your call/boat.