The hot spot is an exhaust crossover that is intended to warm up an automobile motor quickly upon start-up. On a boat it will help the warm up too, but it is much less useful on a boat. In fact, if it happens to be clogged with carbon, I would leave it alone, especially in Florida! Where an automobile is intended to oeprate at ten below zero, the boat never sees that kind of duty.
Here is a thread with photos, regarding the hot spot, which is common on all 427 motors if they are working properly.
If your motor didn't have a hot spot on the intake, it would indicate the crossover was clogged.. Either way, the motor will run just fine without the crossover once it warms up on it's own.
Now that you've verified the system is functioning okay at low speed, you might want to do the same routine with someone driving the boat, to find out what is happening when things overheat. What might stay cool nicely at idle, may show where an obstruction causes a problem as things get hotter. In the end, it may just come down to heat exchanger capacity.