Here's the secret, be very petite or at least the smallest on the raft and you will not get wet.
If you notice, the heaviest part of the raft is the section that gets the most splash. As you start down the incline, gravity turns the raft in the forward direction of the heaviest section (since a round raft has no front or back). That puts the entry point the point with the deepest penetration in the pool below splashing the water up and over that part. You can try and get in the section of the raft with the smaller folk, otherwise it's the luck of the draw. On a hot day in Florida, it's refreshing anyway and your clothing will dry quickly so it really isn't a big problem (unless you are wearing white and forgot your underwear).
One hint though, make sure that you put your feet up on that little foot bar or you will be 'squashing' around for most of the day. Don't ask me how I know that!