You could stop using it for a few days, let it dry some, plug the drain, and then fill the shower pan to see if the water is leaking from the pan.

I mean the cheap water heater is great and that solves the heating water problem. Moving the water to a different building is an entirely different task and if you cheap out there you'll be sad about it later, guaranteed.

Trench and run a proper line to your shop. The water heater (even if you bought brand new) was never going to be the most expensive part of this endeavor.

If it really is only a few times a year, then hoses could work but you'll never want to trust them.

Maybe a hard line to the shop wall and connect the hose outside of the building?