Randy, I'm confused about what you plan to do too. My concern is that you are going to build your spa so you have a solid floor with no drains, the water splashed out will sit on the floor and the water or humidity will get to any wood and rot it out. If the butt end of knotty pine boards are going to sit on a floor surface that is going to be wet often, this is asking for the end grain of the pine to wick water. I suggest that you build a pan with a drain for your floor and put tile or something up 4 to 6 inches around the bottom of the walls.
From what you appear to describe, you will have a small, closed room with wood interior walls and a hundred or so gallons of 100 degree water at the bottom of it. This sounds to me like a recipe for disaster. Cedar would last a lot longer than pine but still will not last forever in that humid environment. Mildew and wet rot will have a field day. Paint will not last but it will feed the mildew nicely. Randy, I am not trying to discourage you from building your spa, but I think that you should check your plans further with someone who has a lot more experience in designing and building spas.