The overdrive of an automatic is a band inside, the load is almost all on that band. Any band inside of a transmission is far weaker than the clutch packs, they cannot take the high loads like the other parts. Towing in overdrive is a great risk of burning up that overdrive band. Don't do it to save a small amount of gas, the damage potential is too high.
Add another trans cooler and you should be fine, that's a light boat. Good luck