I would not assume immediately that anything is wrong with the alternator. They will heat up dramatically when continuously loaded. I would hate to find out you went to all the trouble and expense of replacing or rebuilding yours only to find that it still heats up like that.
Charging at 14 volts sounds normal but what creates heat...other than mechanical problems as you've stated.... is the AMPERAGE that the alternator is "flowing". Boats typically have very low amperage demands with the initial cranking of the engine usually the largest. And that is only a short burst. Other than that, unless you are running ALL the lights and several devices like sonar, radar,radios and pumps there should be very little amperage demand on the alternator.
Before I took it off and had it tested, I would first want to put an AMP METER on the system to see what is happening in my electrical system. If you start up and have all loads on the boat OFF, the amperage should be initially high as the alternator replenishes the amps to the battery that were depleted by cranking but should immediately go down as the battery gets "topped off" and the only load is what is required to energize the ignition system,engine computer and, perhaps, dash gauges. Somewhere well below 20 amps say.
If you were to find that the alternator is constantly being asked to produce 70 or 80 amps with all loads off then there is likely a problem with your system that you should investigate.
An old set of batteries will put undue stress on an alternator and kill it if they have high INTERNAL RESISTANCE. Other power thieves are poor connections. If the battery cables have corrosion or if the connections are loose or dirty...same thing.
The bad connections act...in an electrical sense...just like LOADS and will DEMAND power output from the alternator. All this "fluctuation" can and will occur but will not be seen on a voltmeter. The alternator can be working itself to death delivering amps while the voltmeter may not move off of 14 volts.
Amp testing is easy to do if you have an AMP CLAMP that you can put around a battery cable. It is also a great device for setting a baseline for electrical loads on your boat. It can let you know if you are adding too many "toys" to your vessel that might require that a higher output alternator be installed to keep up.
Just sayin'
Good luck.