Ideally you should have your throttle stick self centring, this ensures your boat stops when the throttle stick is released no messing about trying to find centre off.
Start by turning on your transmitter then place your throttle stick in the centre off position. Next turn on your esc and wait for it to arm (beeping noise from the motor) ensure you do not move the throttle stick at this time. Now see if this has worked giving you forward and reverse.
If there is still no reverse then you need to set throttle range. Manufacturers have slightly different ways for this but more often this will work:
1) transmitter on throttle stick on centre
2) arm esc, wait for beeps
3) move throttle stick to full forward motor should beep
3) move throttle stick to full reverse motor should beep
4) move throttle stick to centre motor should beep
Note on some esc’s you have to set centre before setting reverse
Hope this is clear enough for you to try.