Getting SDC to build it's own resort+moving WW would actually be beneficial for traffic. A number of people would just stay within the resort instead of driving in and out at opening and closing time each day. White Water doesn't exactly attract huge crowds, and most of them overlap with SDC crowds anyway. They would need more parking, or even a parking garage though.
Honestly, a resort is ultimately going to be the main solution to some of their growth problems. You can't do too much to increase traffic capacity through a single intersection unless you just turn it into a highway. Having a single place people could arrive at and just stay put would make a lot of people much happier and more likely to visit routinely. Having to go into Branson itself is a drag for people many families who don't want to deal with the traffic and sprawl.
I don't know if a resort would lessen traffic it would just split it into different times of the day, making it less of a problem. You would get the people who still want to stay in Branson coming in at SDC opening and leaving at closing. Then, you would have the people staying at the resort who would be coming and going at hotel check-in and check-out times.
If they do build a resort and move White Water someday, they would definitely need to get more trams/buses to handle all the people wanting to go to SDC, then White Water, then back to the hotel. Another thing they could do if they built a resort would be to have the parks open earlier and later just for resort guests. Then more people would stay at the resort so they can be at the parks longer. They would bring in more money by having more people at the resort, and if people are at the parks longer they're more likely to buy more food, merchandise, etc.
It would take a ton of hotel rooms to affect traffic patterns either way. This aint no Disney we are talking about here. If a water park was to be built on a top tier level they would need a lot more than daily hotel stays to keep it feasible. And then you have the possibility of another company building back in town and taking that market.