If the boat only has a single motor then place the fuse between the battery and the speed controller, as close to the battery as possible.

The the boat has has multiple motors each with its own speed controller then place a separate fuse between the battery and each speed controller, as close to the battery as possible.

If the boat has multiple motors powered by the same battery and the same speed controller then place a fuse between each motor and the speed controller. Next place a fuse between the battery and the speed controller. The fuse protecting each motor should be a slow-blow fuse sized to blow a bit under the stall current of the motor. The fuse protecting the battery, wiring and speed controller should be sized above the stall current of a single motor, but below the point at which fire in the wiring (yes I've seen that happen on fast electric boats) or damage to the battery or damage to the speed controller occurs. With this seizure of a single motor will blow its fuse but allow the other motor(s) to continue operation.

In electrical engineering this is known as 'Discrimination' or "Selectivity'.
https://en.wikipedia.org/wiki/Selectivity_(circuit_breakers)

It should be realized that staring current of a motor is higher than the running current. A fast blowing has a good chance of blowing under normal operating conditions, particularly quickly going from full ahead to full astern.

I think both Hermank and BobbyN gave an excellent answer.

However, I had a (very cheap) ESC that was defective and already short-circuiting. Luckily, it was a test.
In that case, a fuse between the ESC and the battery would have saved the cables, which had melted all the insulation. Luckily, I had time to disconnect everything and save the battery.
The causes of shorts or overloads are probably more likely between the ESC and the motor because the motor is more prone to stalling (if the propeller jams) and therefore increasing the current intensity to dangerous levels, but I wouldn't rule anything out.
In my opinion, it should be placed both between the ESC and the battery and between the ESC and the motor.

I would use both.

We always place the fuse between the battery and ESC as this protects the ESC. The attached wiring diagram is what we use in our association

Rogal
I always place them between esc and motor and that has never caused trouble