Heating basement floor with water heater
Adding hydronic radiant floor heating is a lot more complicated than you could imagine. To do a system for a bathroom running off of a domestic water tank, in operation for potable water requires all potable water components. Potable water components are expensive. 

This is the simplest picture I could find and it still doesn't technically apply to youwww.floorheatsystems.com

The storage tank isn't being used for domestic water. It's only being used for floor heating. 

Other things that you need to consider are legionnaires bacteria radiant floor temperatures are right at the bacteria's ideal growth temperature of around 115 degrees Fahrenheit. 

The Solution
Electric radiant floor heating is your best bet. 

Resistive electric floor heating mats are available www.findanyfloor.com

This requires a little bit of electrical but is much simpler to install. It's also much thinner then pex.