Does the repeated turn in of water/food/champagne (etc.) result in anything significant?

You can turn in food and water (and I guess alcohol/champagne too, but I haven't turned in a lot of these) but there doesn't seem to be any end to how much you can turn in. So far the reward has just been cash each time.

Is this just your standard fare repeated-quest turn in, essentially giving water/food a higher "sell value", or is there another tangible benefit to providing these items en masse?

From my experience in game there is no other result to these quests other than monetary and small experience gain (like around 50 exp per turn-in).

Though I will say that if you find any champagne, turning that in is a pretty sweet deal. You receive a diamond that you can sell to merchants for around 1500 dollars. And you get one for every bottle that you retrieve. That's the best reward that I've encountered so far.

There are some mods later on that use some of the rarer components like the Diamonds.

Alcohol is probably best saved to swap for Molotov Cocktails from Harlan Johnson (3 bottles per Molotov) or for your own creation once you have the recipe (2 bottles per Molotov), as Kurley points out below.

As for the other repeatable quests I don't bother after the first one except for the Stanley one that has a few iterations with better XP IIRC.