Hi John, Agree with Andy, Hobbyking have a massive choice, I use go-market myself. Cant help thinking 400mah will be too small, your stated amp draw on the motor looks very low, have you got a link to it? Is the boat a fast set up or not?
Sub 1000 mah lipo tends to be used a lot on very light micro helicopters, and planes, and the run time is generally 8 mins or so. If you imagine they are more efficient with less amp draw (no water to move, just air) have a big gear ratio, and small brushed motors, such as in the esky lama coaxial, these have minimum 1000mah lipo, and some slightly larger or heavier such as the twister skylift chinook runs a 3 cell lipo.
I have just finished a jet boat project for me and sons. I tried a 1500mah 30c 3 cell lipo, it struggled a bit, now running 2300mah 3 cell 30c, run time is 10 mins. I will in the next few days fit an amp meter for you. it might help with your decision.
Paul