Conduit size for UF cable
The answer is yes, you can run UF in conduit. For your short section this would be normal. I normally use 1 inch; it fits easily and the NM is not damaged. UF is slightly thicker but should fit it depends on the MFG; some are round and some are flat, 3 wire or 4 wire, etc. A water heater only needs 3 wire normally: 240v and ground with no neutral.

I usually have to have a section of metal flex conduit from the wall to a water heater to protect standard NM since smurf tube or non-metallic is not allowed and liquid tight is too expensive.