Fortunately all my seacocks are accessible from inside the cabin area.

The description however of how you access the engine is quite familiar... I sit on the water heater and have access to the engine, transmission, muffler, fuel filters and stuffing tube, and yes, once in that position, you don't want to be hopping up and down for tools...

In your case, having to go in and out of the manhole indeed would be the biggest labor factor... like trying to crawl in and out of a Bomar-style hatch, no doubt. My case is easier... just trying to "walk over the missing deck," granted far far easier than that manhole, but still a balancing game. GRIN