H2 Toing a 9000 lb. boat?

[Big Z]

"purchased one of the H2 reprogrammers that has an option for tow mode"
I'm not sure what you mean here, unless your reprogramming the tow Hual mode (basically shift points) in your H2.

"The longest trip I would be making is 5 hours... does that make a difference? what type of trailer would be best to take off the strain?"

Flat and level, you will probably be fine, hilly terrain, watch the Temp guages. As far as the trailer--What did it come with? Triple axle, with surge or electric brakes? balanced for correct tongue weight? What ever it is, the loaded trailer Needs to be able to stop it's own mass.

Exceeding your vehicles tow rating, isn't a good Idea, there are reasons for them, plus the Risk and liability is just to high. (in my opnion).

It's A matter Safety, and the Right Rig for the Job.

In the top pic, I was pulling out of a truck scale. Knowing your numbers Is a must! Guess work be gone!