I have seen folks put tubs on the roof of houseboats plenty of times, and two things always come to mind: doesn't it make the boat tippy, and do those things really have the structure to handle that much weight on the roof?
I have put a few tubs on boat, but always on the deck, and even then there were phone calls to the builder to be sure I wasn't going to break something or sink something...
Looks great, BTW, and your caption sums it up very well.