Teri--The overdrive debate gets pretty crazy sometimes.My feeling is that as long as your transmission is not hunting between gears,you should be alright.I agree that on the highway at 70 mph you can use overdrive with no problem.
I have a smaller boat,but also a smaller motor(v-6) and at 65 mph I get much better gas mileage with the O-D off.It keeps the RPM's up in the range where the motor makes more power,so it dosen't have to work as hard.
Next time you make a long pull,try it with the O-D off,and on and see which way is the best.