I don't know if cistern would be right, as the idea that they are naturally formed, not built. i think I mentioned in the story that they were filled up by heavy rains.
Then I guess the technical term we are looking for is "puddle," or perhaps, "really big puddle." I think that sounds nice, don't you?
Really Big Puddles and Grainaries Aw yeah.
As for naming the follower played on the water feature... With the story you have now I think you should stick with "boatman" or change it to "boater," since in the story the water is only there as a result of the rain and the followers are there just for the fun of it.
If, however, you want to change the story, and say the people built cisterns (or reservoirs) to guard against drought or something, then it makes more sense to call the water feature in question a cistern, and the follower on that feature a "mason."