This job can be done most of the time, access permitting, from bellow deck with a rechargeable impact driver!
I have re-bedded LOTS of fittings solo with my simple and inexpensive Ryobi 18v impact driver. Honestly it works like magic! Brrrr, brrrr, brrrr and the nut comes off with no wrench or screw driver being used on the deck level! Brrrr, brrrr, brrrr and the nut goes back on with no wrench or screw driver on the deck side.. Be careful because with some of these it's very easy to over torque and snap a bolt clean off. The "IMPACT" effect allows you to tighten from one side in most cases..
Or a slightly higher quality tool with a lifetime warranty: