I had one of the original cars and you used about 10 rubber bands for the propulsion system. I never had trouble with the nose falling off.
If you thought it worked well in a bath tub, you should have seen it in a swimming pool. You did need to rinse it off well to get rid of the chlorine. I think the weakest part of the toy was the foam float. It was open cell (like stuff florists use) and tended to break down over time. Good news was replacement foam was easy to come by and easily replaced. NOTE: there were two versions of the sub model, operational and non operational. You could also buy the sub kit as a separate item.
The sub model was actually pretty close in design to a real Navy sub, the X1. These are pictures of the sub being delivered/installed at the sub museum at Groton CT. I don't know if the Navy ever transported the sub by rail during it's service career. The photo's show it being taken off a heavy duty flat bed that delivered it to the museum.