I did some rough thruster tests which you may find interesting.
Brushed vs brushless motors - General Discussion - Blue Robotics Community Forums
scroll down to my test setup and be aware that the plastic tub I used would not hold as much water as I wanted. The sides will bulge out and you will need sand bags or a frame to hold the shape.
It uses a lever pressing on a food scale to measure the thrust. Do the math for lever length, etc.
Note that thruster efficiency can differ a lot from what you think. Smaller does not mean a power saving.
I use Cat5 tether with 4 pairs of tinned and stranded wire. Not the cheap non stranded (solid wires).
One pair is used for bi-directional Ethernet (BlueRobotics/Ardusub style) and I am reserving another pair for my custom bi-directional communications to avoid messing with the Pixhawk software. The other two pairs can be paralleled and high voltage sent down. I was planning 200 v DC.
In your case 48 volts. My Cat5 uses #24 awg at 0.027 ohms per foot (x2 = 0.054 ohms per foot for the pair for a total of 2.7 ohms for your 50 ft tether, and half that if you double up a pair. If you don’t need the extra pair it would be roughly an ohm for 3 pairs). 6 amps drops 6 volts from your 48 volts. You will get close to your 24 amps but your average current will be much less. Are you allowed to use a large capacitor? Are super capacitor available at that high a voltage? Just a thought.
1752A Technical Data Sheet (belden.com)