I name..3. For beginners.
1 from lakes, 2 from rain...and not given out to some random alt, but someone who'd actually agree to trade the water fairly if needed and not abuse it.
I'd put a better limit on the lake one, so they get a regular, big harvest if they do continually do it..but however, the rain can suddenly create bursts..different magnitudes of the gathering could be interesting.
The best getting 3 water from rain, instead of just 1?
The biggest issue still lies within the fact that we don't know how much water will be needed in future..if it were me, I'd put 3 out now, and add more accordingly as the holder abuses it/doesnt use it/etc. I suggest a contract of some sort. You give out, loan the tools. If they don't do their job, you can confiscate the item and something they agree to put as security. In return they get to enjoy the profit of collecting water for trades...which should benefit them.