Rooster, I tow a lot with my Aisin equipped 3500. On the hottest days, with really heavy loads, if you keep your speed at "normal" levels the temps should stay below 200. Today I hauled a 26,000lb trailer out to a rig location east of Denver. Air temps were in the mid 90s, terrain was flat with rolling hills. Nothing to really slow me down. transmission temps were usually in the low to mid 190s. In the past with much lighter loads on colder days hauling up a steep dirt road, traveling at less than 10mph I could quickly see temps well over 200. If you keep the air flowing over the transmission cooler the temps stay down.
On one of those slow steep hauls I did get the transmission hot enough to get a warning buzzer and idiot light. I don't remember at what temp that happened but the truck didn't go into any kind of limp mode. I just pulled to the top of the hill (it wasn't far) and let the truck relax a bit heading down the other side. It cooled pretty quickly.
I cant imagine you will have any problems. My empty flat bed weighs double what your boat weighs weighs!