Yup.. similar problems in RC submarine hydroplane operation..you want the actuators outside the pressure hull fully immersed..but the main gear inside.
I don't think a servo tester should draw more than a few mA by the way..its likely to be a 555 chip and not much else. All it is is a pulse between about 0.5 and 1. (or is it 1 and 2) ms every 20 ms or so..a realaxtuon oscillator with adjustobale discharge is all it takes. Heck a unijunction transistor, and 4 resistors and a capacitor would do it..