A good motor will have strong magnets which will give a very definite "feel" as the armature poles are pushed past the magnet pole faces.
Sounds from a motor are usually caused by the ESC. Resistance types are silent, old ESCs that ran on transmitter frame frequency (typical 50Hz) would rumble, mdern ESCs run at higher frequencies and cause the motor coils to whistle at that frequency. Generally speaking, at full throttle they go quiet.
An unexpected screaming noise sounds like a lack of lube. Could be a dry motor bearing, could be a prop shaft bearing. Could be that the grease in the top bearing has been replaced by water. Broken brush gear usually shows up in bad performance.
Water entering up the shaft tube usually sprays a line of muck across the hull level with the top of the shaft tube, unless there is something to catch it.
Gunnel holes should let water off and should have their lower edge lower than the deck. I have had a problem in the past where the deck at that spot wasn't as glued as I thought, so some that should have drained away drained in.