Every year a friend of mine and I compete in a wacky boat race. This year we wondered if we could build a boat using the mechanism of a handcar railcar to power a propeller. My question is would 3D printed gears be able to support two guys working a hand pump powering a propeller underwater? If so, what ratio might the gears be able to support? I have no engineering or printing background but after reading a few threads here I thought this would be a good place to ask.

Here is a video of a handcar driving mechanism
https://www.youtube.com/watch?v=VDdp8sh4G3U
The difference in a handcar and our hypothetical boat is that we would be oriented east and west on the boat rather than north and south like on a handcar as we would need the second gear to turn a propeller shaft behind us rather than an axle and wheels to our side.

Thanks for any help!