ESC Specs from the Proboat website:
Continuous Maximum Current: 40 amps
Input Voltage: 4-10 cells (4.8-12 V)
Motor Limit: 15 turns
Dimensions (WxLxH): 42 x 47 x 13 mm
Weight: 89 g
BEC Voltage: 6.0VDC 1 amp. Max.

Someone with who may have "tested" the actual limits of this ESC might be able confirm what the practical limits are, which are likely to be less than the advertised specs.

The internals of the Apache hull definitely limit your battery options. A 6 cell pack is pretty much the limit unless you modify the area where the pack fits or if you go lipo. Lipo is probably the best bet if you end up wanting significantly more power.

I'm considering a mild upgrade from stock.