Here is the logic I used.
Frist off, is the initial reason for overheating an unknown? This is very odd especially for a boat only a few years old and only used in freshwater. Also, the impeller melting is an oddity as well. I have never heard of them melting, breaking apart yes, but not melting. However, my experience is limited in this area. Pictures of the melted impeller if you still have it would be interesting. And you replaced the impeller yourself? Point is that the RWP seems to be the initial culprit and perhaps remains so.
Now for the other part: these RWPs are a flexible impeller vane type. That means it’s a positive displacement pump and water should not pass from inlet to outlet unless the impeller is not sealing (i.e. its broke or somehow has excessive clearance between the impeller vane and the housing. Thus a ‘non-sealing’ impeller would not pump as expected, leading to overheating. When you hook up the garden hose you have between 40 and 70 PSI of pressure from it. Again, the pump is not pumping as expected but the hose water has enough pressure to get by the impeller but only if you are blocking up the drive’s inlet.
Thus, this logic points to the pump. I get that it sucked in a bucket of water and cannot account for that. But here is a statement: Everything appears to be up to snuff but it overheats, therefore something is wrong. Don’t shoot me for this oversimplification, but I am back to what I do when nothing is wrong but still does not work. My recommendation: pull the pump apart and post lots of pictures.
I'm sure it had been running hot previous trips before without tripping the alarm damaging the impeller enough to lead to a near complete failure at start up that last time and throughout my diagnostics.