We were just there on Saturday!
Yes, you can bring wagons - in fact I tripped over someones. They also rent wagons, but I don't know how much they cost b/c we're all on our own foot power now. I don't think you can lock them up at all. I didn't notice anyone leaving them unattended.
As far as the cabanas, depends upon why you want one. It wasn't hard to find a place to park our towels, but there were a lot of towels occupying the beach chairs (even though hardly any people were). We didn't check into the cabanas b/c we only spent a couple of hours in the water park.
When you rent tubes, you are responsible for them, but they have wristbands. They come in handy on the water slides that require tubes b/c you don't have to wait for one to become available. They have tubes and rafts available for people who don't want to rent one for the water slides, but not the wave pools. Most people in the wave pools didn't use them. When you turn in the tube, if you are wearing the writstband, they'll give you a dollar or a piece of candy. I didn't notice anyone having a problem with their tube being swiped, even when tubes were left unattended.
If you want to know anything else, just ask. I'll try to check back tomorrow.