Firstly you should appreciate that a drain on the Rx battery like this will discharge it faster than normal, so you will be able to sail for less time until the radio stops functioning. It would also introduce a very "dirty" component into a circuit which is already sensitive to RF noise. All in all, not a good idea unless the pump motor is very low-drain (unlikely) and heavily suppressed.
That said, for any solution to be offered then you will need to measure the current which the pump is taking under load (i.e. when it's actually pumping water) and to confirm what voltage the test battery is, and then come back to the Forum with the answers.
If the pump is pulling more than about 250mA then it's certainly not advisable to run it from the Rx battery. If it's less than 1A then you could use a separate BEC regulator to drop the main motor battery voltage for the pump. Any more than that and the best solution would be to use a separate battery for the pump, of the correct voltage, and switch it from the Rx via a simple relay switch (e.g. ACTion P43).
FLJ