When were there on 6/8 there were some single riders on the family slides. They were put with others. But, they were not crowded. For example, the water coaster holds four and there are seat backs, but your legs need to go forward and touch the person in front of you. They put a single rider with a party of two and then left a seat between them. The other raft rides hold four or more and again you should feel comfortable going with another small party.
I rode some stuff as a single. No problem. In fact, I got to cut the line a little sometimes to fill spots. I would try to get a locker, since you won't have anyone to watch your stuff. Unless you don't have any stuff to steal. Have fun!