Is there any problem with towing a 1800 lb boat in over drive about 70mph? I dont want to screw up the transmission, and about to take a rather long trip. Also how do you know when to kick it out of over drive?

 

as long as you're not lugging the engine (think trying to go uphill at 1,200 rpm), you'll be fine. The tranny will downshift when it feels the need to.
If you see it constantly shifting between gears, lock OD out and stay in the lower gear.