You might have an LVC (Low Voltage Cut-off) setting on your ESC that doesn't match battery's voltage. If you haven't already, read the ESC manual and see if there is a way to set the LVC. You might also want to re-bind the receiver to the txmitter. If your battery is charged and LVC setting is correct, you could have a bad battery or bad ESC. If you have a way to get a 2nd battery, it wouldn't be a bad idea to do, anyway. I realize that you are probably no where near a LHS and you want this fixed, ASAP.

As Jworm states, you are better-off with a real charger. This will allow you to balance charge, and you can see if one of your cells is bad (I'm assuming that the charger that came with the boat is some sort of wall-charger). You can spend as much as you want on a charger, but you can get a Thunder AC6 for $40 - $45. It isn't a top of the line charger my any stretch, but it does everything I've described, and it comes with an AC power supply (you have to find your own AC power supply with many chargers).