Another Riva Aquarama, excellent decision.
I haven't actually used the G2 Hydra 30 units, but on the face of it / based on the specs they look well suited to the Amati kit, I do think you'll need both units. In the same boat I'm using 2 x 3530 brushless (same 1100kv as the G2s) on 3S LiPos, restricted by the transmitter to 80% output, and they're still a bit overpowered and draw 10A maximum. I think if you put PropShop 1415 propellers on those G2 motors they would work well. Given the G2 motors should rev at similar speed they may use a but more current to spin those props at full speed, but I'd have thought well within limits, and most time is spent at lower speeds anyway. I see the motor sizes are smaller in diameter and longer in length (compared to usual outrunners) and this will help let you keep the motors low in the hull, so better prop shaft angle too.
If you went for LiPo power, I'm not 100% clear whether the Hydra 30 ESC units can handle 3S liPo's as they exceed 12V when fully charged (12.6V ish), I'd have thought they would be OK but maybe someone else can clarify..? Running the boat on 2S would reduce performance quite a bit. LiPos help keep weight down and power up but I know some folks don't like them.
My main feedback with this kit is that it builds heavy as it was designed as a static display model. Building for RC needs weight saving to be taken into account at every opportunity, especially up at the bow.
Look forward to seeing the build take shape..!
Best regards
David.