@Hsailer -As I mentioned, 2 to 3 second shots, just long enough to get an Amperage reading.
I have been working and testing motors for over 50 years, snd have never fried one yet by testing.

@GregHiltz
I wouldn't hold the motor "still" for long. You could burn it out.

Wow. I never thought of any of this stuff ..... back in the day.
But it all makes sense. I will delve in again after 15 years of unfinished hulls.

As said by another member, you need to determine the max of stall amperage draw.
You did not specify if single or twin motors in your Tug, but assuming single, I suggest putting a pair Vice-Grip locking pliers onto the shaft, and then place an Amp-Meter with a minimum 10 amp range, in line with one leg of your motor lead, between a motor lead and the corresponding motor terminal.
Personally, I would do this to the motor alone, not running with the R/C controls, hooking motor leads to Batteries through the Amp Meter only.
Make your final battery/ Meter connection in short bursts, eyeballing your meter reading. Do do only 2-3 seconds at a time, ant this will give you the max draw from the motor, replicating the motor seizing, or the prop getting wound up with weeds or rope.
The 12 pole motors I have used in some models, have been very efficient, stalling at 3 to 4 amps.

@merchant67
The fuse is going to depend upon the current draw of the motor. That will be determined by the motor windings, the weight of the model, whether the motor is geared or not, and the prop size and pitch.
One way to cover you bases is to place the motor in a holding device, like a vice, but don't crush the motor. Place an ammeter in series with the motor leads and, here is the tricky part, you want to determine the current draw when the motor stalls, ie, with current flowing the shaft of the motor is held immobile. This should be done some that it is a momentary reading and load on the motor, otherwise damage can and will occur. That current reading will be the stall current and can be used as a guide to determine the fuse, or circuit breaker size, for use on your model. Your choice, go either side of the reading you got.
Folks will advise either way.
I personally prefer to use auto-resetting circuit breakers because they will reset and hopefully allow you to get your boat back. Fuses are generally more reliable, but one time use only, AND, you need to have a way to get your boat back to shore.