Best bet is to fit a fuse and try it in water so its under load. Auto type blade fuses are available at low cost in little boxes with a variety of values - either start with a high value and work down until one blows, or start low and work up. The lowest value that survives is the one to go for. Pick the next rating up for the ESC, that way the motor will have the right ESC, and the ESC will be protected by the fuse. Or just measure the loaded current with a meter, not always easy reading a meter and holding the boat still.
The loading will depend on the motor itself, condition of the motor, condition of the drive train, prop diameter, blade count and pitch, so a definitive answer is not really possible, but you are likely to wind up with either a Viper 15 or 25. I have run a Graupner Lotse with a speed 400 on 7 cells (its rated voltage) on a Viper 15, no problems, apart from looking a bit silly for a pilot launch with a mast and crane when the front 2/3rds was out of the water.