I didnt like the height of the box he used for the gtp. IMO that was just too short. I have seen a water based cage before. A matter of fact the GTP that I own came from a guy that kept him and a ETB is such a cage. The GTP came to me with an RI, but thats another story but the cage was a factor.
I didnt like that setup then and I still dont today. After owning my GTP I have learned things from him that tells me a water based cage is a terrible idea. My GTP comes down almost every night and crawls all around the cage floor. He will also sometimes coil on the floor and just relax a bit before morning when he takes back to the perches.
He also regularly swims in his water bowl. So I do think having a large water bowl is a good idea but not one that takes all the floor away. I also dont think a gtp is a bad swimmer. From what I have seen from mine he can swim just fine.
Now having a ETB as well I have been seeing very similar behavior from her. They may stayed perched most of the time but they do come down to the cage floor almost nightly. The only time mine usually dont is during a shed. At that point they dont move until the shed is complete.