It would depend on the particular power vent water heater
requirements. If you already know its brand, check online for the manual / installation instruction and verify PVC vent pipe and motor assembly clearances (or let me know and I try to find it for you ).
It would also depend on the PVC vent pipe diameter requirements but I believe you should be OK, Important things to consider for the water heater crawlspace installation is sufficient combustion air supply, so it burns without producing Carbon Monoxide, and possibility of moisture / flooding. Let me know if you have any other questions.