Very little. The Bronco is a bad choice for a tow vehicle. Most every modification done reduces what little towing they once could do.
Like many hear I have towed, several times well over what it should have done. Had plenty of close calls in what little towing that I did do. Lets see the short list...
Brake fade with the drum brakes on 33s trying to slow down to make the turn at work. Coasted a little too fast through that left turn. Good thing there wasn't a panic stop. Did disk upgrade after that.
The trailer pushed the back sideways while stopping. A little jack knife that stopped before stuff got wrecked.
The panic stop at the forgotten stop sign that had the brake bias too heavy on the front and I blew that stop sign pumping the brakes like mad trying to get it slowed down in a panic stop.
The flat towing another truck and having it pull me into a ditch I won't count as it really isn't a trailer.
In short I have exceeded the towing capacity multiple times and gotten lucky. Most of that was done with about a 2000 pound trailer and only 33s. I sure had the power to pull it great. But the stability lacked. And it is hard to judge stability until you find you have gone over the limits.
I will admit that a Bronco with power steering is about as good as you can get for maneuvering on a boat ramp. You can really stick it where you like. And it will pull like a ramp like a champ, 4Lo and with low axle gears will get the most overweight pig out of the water. And as long as you are at tractor speeds, everything is great.
How big and heavy of a boat are you looking at getting?
How is the Bronco set up?
How far do you have to drive it?
Just from how you ask the question, I am already going to predict my answer. No, don't do it. Not safe and you really risk balling up all your toys at once.
Some exceptions. A little Jon boat, few hundred pounds, probably OK. Store the boat at the marina and just need to move it to the ramp and back, pretty much any size boat is fine.
But if there is any highway involved, not a good plan.
For reference I have actually done some work that involved testing and certifying towing performance for SAE J2807 test standards. A little more qualified then the average Joe for what can be done, or should be done that involves towing. Not that I know everything, but I know some good engineers that can fill in the blanks that I don't know.