"Is BEC necessary for subs?"
1. Consider what BEC stands for - Battery Eliminator Circuit. When you consider just how tight most pressure hulls are, where would you put an extra battery?
2. Do you have any idea how frustrating it can be to find out AFTER you get to the lake that you charged the transmitter and main drive battery but you forgot to connect that 3rd charger?
3. Generally speaking, your airplane scenario isn't appropriate. At its slowest speed (when using a speed control), the airplane motor is drawing much more than a sub draws at its highest optimum load. Put just a 10 amp load on a 1700 mAh nicad and look at the voltage drop at the battery. That same load on a battery that's at end of charge won't have enough to drive the receiver/servos.
4. While there are some boats running on 6 volts, most are 7.2 volt and higher. Since receivers are designed to run on 5 volts, when you see your boat running noticably slower, there's still plenty of juice left to run the motor (at slow speed) as well as the receiver/servos to get you back to shore.
But, as valid as that question is, it seems you're thinking that it's the BEC which cuts the power at a certain discharge level. Not true. By itself, BEC takes the place of the receiver battery. (See #1 above) There's another circuit which senses voltage and cuts power to the motor when a certain threshold has been reached.
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