water pumps are one of those items where they are not "made" aftermarket (usually), rather remanufactured.
Who does the reman process, what is done, that is the question? And what components are they using?
Often reman parts are sold at a reduced rate than OEM new replacement, which it sounds like the OP is looking for....
with that said, ask yourself why it's cheaper. I have had exactly zero luck with reman water pumps, except automotive industry, but I generally rebuild them myself if possible using known-good quality components.
I have also had reman automotive a/c compressors die within 50 miles of use, due to failed bearing(s). In one case it never made it out of the shop, turned the engine and a/c on, ran for about 5 minutes checking it over and the compressor started making noises, seized, ate the belt off and filled the entire system with a/c failure "stuff". Reman, so the only thing "they" (reman company) paid for was the compressor, while I was on the hook for everything else (about $2500 my cost). Never again. I'll still offer reman stuff in some cases, but when I do I let the owner know what the reman warranty is-the reman part only, and nothing else.
On my car, I rebuilt my own a/c compressor. Piston type, pretty simple about like an HST transmission. A lot of Kubotas use scroll-type compressors now which are a LOT more efficient, also a lot more expensive. Generally speaking scroll compressors don't die, UNLESS the system is not taken care of. I.E., let the condenser get plugged, evaporator get plugged, and/or let a non experienced "a/c tech" do some work on it (under charging, overcharging, etc) then they can fail.
In Kubota stuff, we sold a few reman starters and alternators. Failure rate was roughly 75%. Warranty didn't always take care of it either. We (dealer) quit selling them. Things may have changed since.