Can I use a solar thermal water heater to heat my pool in the summer and the floor of the garage that I am going to build?
...pool.
Yes, you can use a thermal solar water heater to heat a pool. The question is are you looking for a commercial solution, meaning a thermal solar panel to buy, or were you looking for instructions on how to build a solar thermal water heater?
There are commercially available solar pool heaters, like the Vitosol ...