I keep water in with my rosys too. Can't say I've never had regurgintation issues, but they were stress related and happened shortly after I acquired them. Now that they've had time to settle in they're doing fine, water and all. Granted, I live in a dry area, so it's easy to keep the humidity down.

This is just speculation on my part, but I don't think the water causes regurgitation. I think it makes the regurge less painful. If you were a stressed out snake who was about to lose your lunch, you'd think that drinking a bunch of water would dilute your stomach acid and make it come up easier. I've seen rosys regurge with water and without, but when they drink water beforehand the regurge doesn't seem to look or smell quite as bad. They also seem to recovered faster after a wet regurge than a dry one.

These are just my observations though. I don't have a lot of experience with rosys. I've only owned mine for a few months. So I tried it both ways, because there were reputable people on either side of the argument, and I found that keeping the water in with them seems to work for me and my snakes. It might be different for others. I'm sure the humidity in your area would factor in, and maybe even the species/locality of the rosys. Mine are Mexican, but non-locale specific, so I don't have any info about where they're from, unfortunately. They came from private owners and pet stores who weren't terribly knowledgeable about them.

Hope that helps! You can always try leaving the water in with him and take it out if there are any problems. As long as humidity stays low I don't think it will be an issue.