While not on a boat, getting at the top bolt on the tranny of an older Western Star requires some serious contortions, and someone to extract you when the procedure is over. You have to block up the cab, then pull yourself in from the rear using your finger tips. You have to use an open end wrench because there's no room for a socket, and a box end will get locked in when you back off the bolt. When finished, your "accomplice" has to tug you out by your feet.
I find whistling the theme from Mission Impossible is helpful.