You might find that you had your throttle stick in the fully down position when you switched on for the first time. The ESC may well "read" this as neutral (i.e. motor stopped) and will allow forwards to max throttle as you push the stick up. However, of course there is then no scope for reverse, even if the ESC has that capability. You need to "teach" your ESC that neutral is in the middle, and that one way is forwards, the other reverse. Without knowing the details of your ESC it is difficult to give clear instructions, but you could try the following -
With everything off, switch on your transmitter and place the throttle stick at the midway position. Connect your ESC and immediately push your throttle stick to max forward, back to neutral, then max reverse. Return to neutral. You might need to switch the ESC off and on again now. When you switch on again, wait for any tones to stop sounding and try your throttle again. If that doesn't work, you will need to do some more research on your ESC (or tell us the model and someone on here might be able to give you detailed instructions).
(Just a note - I confess to using the left-right movement on the left stick for throttle, because that is sprung and finding neutral is dead easy - just let the stick go. It also means if you drop your transmitter, the boat stops rather than haring off into the distance. Or you can fit a spring to the up-down movement.
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Good luck
Greg
