Water Storage Tank Made from Sewer Pipe

Personally, I would look at putting a jerry can holder inside the float, away from the horses hooves, and use a plastic jerry can designed for water.

I have to agree..... I use a drinking water quality plastic jerry (the blue ones) that carries about 20L of water. I put it in the front tack box of my float whenever I am going somewhere where I am not sure there will be easy access to water. Having said that, I have only had to use it at one place.
If you want, you can get little taps that fit into the bottom of the jerry. You can also get water quality jerry cans that hold more than 20L, but they get heavy to lift!
If you can't put a jerry in the float (away from where horses could knock it), I'm sure you could put it in the car somewhere??
Cheers
Leanne

Rule No 1: Don't buy used pipe. :P