You can drive and tow with your truck in OD
with no problems. The only time you need to shift out of it is when the transmission is hunting for a gear such as down shifting and up shifting back and forth. Other than that just drive in OD
As for speeds, how heavy is your boat and are there any restrictions in place for speeds for vehicles towing something? Other than that just drive at the speed limit if you feel it is safe. Just don't lug the engine down.
Don't over complicate it.
96 F250 XLT 4x4, 4:10 gears, 5 speed, Tymar intake and HX hose, downpipe, pyro and boost guages, coolant filter, Luc clutch