Being a two part question I wouldn’t really say yes to using it for both, but let’s start with the 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 solar thermal panel from Viessmann; we like this one in particular because it has its own ‘built-in’ overheating protection.
As for directing solar-heated water to your garage in the winter when you aren’t using your pool, that would significantly complicate matters as you would need a glycol mix for your panel to avoid freezing in the winter, and you wouldn’t want to swim in that. I would recommend you check in on the Viessmann company page (or others) to check how their panels work for pools.
The proposition of using one panel for two separate seasonal uses is not impossible, but it would likely be an awful lot of work and maitenance. I think you would need to include a liquid-filled tank to use as a thermal battery heat exchanger, and run both your pool water and floor glycol through it in copper tubes. Alternatively you would probably need to remove the glycol in summer and flush the system to put it to use for heating pool water. There are for sure people out there that would be a fun weekend activity and have the skills to pull it off, but that's not everyone.
So I would propose you look for heating solutions separately -
Commercially available solar water heaters are great, but they don’t come cheap. On the DIY thermal solar panel side, an affordable and relatively easy solution would to build your own solar water heater with a coil of black PVC Pipe.
That is a rather simple matter of having a coil of tube facing south to pump water through and then return to your pool. How ‘easy’ this is depends on your plumbing skill level, though I’m sure you could find a local plumber willing to whip up a makeshift system for much cheaper than you would be able to purchase and install a panel for.
DIY solar pool heater
But a solution like that wouldn’t necessarily be a useful option as a solar floor heater in winter given the risk of freezing, also that it would likely hold snow, rendering it useless.
As for your garage floor, it really depends on how much heat you are looking to inject, whether its just to melt snow and keep it above freezing, or if it is intended to be kept at a comfortable temperature of humans. I would point your attention to the solar air heated radiant floor that we just built for our new demonstration house. it would be easier I think to provide solar heat to an air heated floor than hydronic.
Below are a few more pages that may help you sort this all out, good luck and please let us know what you finally decide on. And if anyone else out there has other ideas please let us know in the comment section below!