How long ago did this happen? If it was more than a day or two, (you said "one day") and it was in the rinse tank for several minutes, and you simply put the dust cap on and put the reg away, a rebuild is probably the best idea.
For the future, if you suspect that water might be in the first stage, like if you look into the rinse tank and see your reg in there with no dust cap, you can try drying it out with tank air, but you should do it immediately. To do this, take all hoses off the reg, then plug all the LP ports, leaving both HP ports open. Run a few bursts of air through, make sure there's no water coming out the HP ports, and wait while the HP air dissipates through the ports. Then plug the HP ports, unplug all the LP ports, blast a little more air through, in short bursts. A lot more air is going to flow; you won't be able to leave the valve open for more than a few seconds without everything getting really cold. Then, if you're really paranoid, attach a 2nd stage to one LP port, plug the rest, remove the SPG from the HP hose, attach the hose to the HP port, blow some air through it. You can purge the 2nd stage to release the pressure quickly once you've turned off the air.
Then, finally, attach both 2nd stages, re-attach the SPG, and pressurize. Check the IP if you have access to a gauge, make sure the SPG works, take a few breaths and purge each 2nd stage.
That's about as much as you can do without tearing the whole thing apart. FWIW, after the last salt water dive and subsequent rinse on a trip, I almost always take the HP hose off my reg, put a plug in, and run some air through my first and both second stages.