On Roaring Rapids, I got a poncho to wrap my bag in and held it but showed it was looped securely around me as well. That seemed fine. They sell disposable ponchos in the line for 10 RMB.
It's random how wet you'll get. MOST of the time, you barely do, but since it's a raft ride, you can hit a bump and splash the right way to get super wet. I had my lower right side get DRENCHED that way, but I honestly don't think the poncho would have done much to prevent that. I also suspect there might be some water features that they turn off and on, depending on the temperatures, but I'm purely speculating on that.
If you're going in the summer, there may also be a "splash" element to the Castle Stage Show. My friend and I did that and got soaked. I think, again, it's "saved" for the hottest days where you're likely to dry. I wish I had been wearing a poncho then, though.
Tron now has these carts that you put your bags in and they roll through to the exit for you to pick up. It's monitored by staff, and does seem to correspond well to each ride, so it's hard to for someone to grab your thing from an earlier ride. It's still possible, I'd guess for someone on your ride to go quickly and take your stuff, but I felt pretty secure about the whole thing and would imagine there's cameras and other prevention for stealing. I think this has significantly cut down the line time. I went in the days of the lockers and it was a madhouse and really unpleasant. The blue cart is MUCH easier and efficient. There is a small compartment on the bike, but it's mostly for hats or glasses and sized appropriately. i put my glasses in there and they were fine.
Right now, there is an undated "bounceback " ticket offer. You may look into that, if you're going for 2 days but not consecutively.
