The water around the island is a good deal deeper than 6 inches. It is about 8 - 12 feet deep in the center of the "river" (that's what it's called - the "Rivers of America" In the center of the river is a large track that the paddlewheeler rides on.

The bottom of the river is concrete (which is not unusual in the LA area, but here in FLorida, quite odd. I've been at Disney World when the river has been drained so that workers can clean the concrete and work in the mechanics.

It's like the famous tunnel system underneath both Disneyland and The Magic Kingdom - they're technically not tunnels. Disney cleared the land and built the tunnel structure and then built the rest of the parks on top of them. The street level is actually the roof of the buildings. The buildings "above ground" are the upper stories. (and by the way - the ones on mains street are faked in another way - They appear to be 3 stories. In actuality - the two upper stories are both shortened so they aren't as high as they appear. There is only one story above the ground floor. That second floor (1st if you're British) is offices for the Merchandising and Security divisions.

But I digress . . .