If you don't bring it back you will hit a cut-off and the boat will stop. Generally when this happens the voltage will recover after several minutes and you can idle the boat back to shore. This is not recomended because discharging the batteries this far will often get them very hot and damage cells (I have this problem more with my racing packs than my sport packs). It's best to watch for the performance drop and bring it in pronto. With Lipo it's a whole different game - use a timer or get a good LVC (or both)