I have an oak whiskey barrel that holds water quit well after the wood gets wet and swells. There is no fancy joinery involved just straight and paralel boards that are butted against each other. This should work for hot tubs also just make sure you keep the outside circumfrence stays the some which you probably wouild have to do any way.
It s sized for 1/4" to 1/2" wood...great for strip built kayaks and canoes, but your hot tub will have to be way thicker than that. I think hot tubs, and wood water vessels in general, are made from staves that have plain sides = angled based on number of staves in the circle.
I have used the bit set pictured...worked great, but remember, in strip built we are not looking for water tight. The epoxy layered coatings give us that. I doubt if you will find a router bit that has so slight an angle that you need. You can make an angle jig for a straight bit or adjust a jointer. And remember, 180 divided by the number of staves will give you the angle.
John Lucas www.woodshopdemos.com