The only reason this is a trap is due to the amount of people. In the summer and good weather there are tons of people from Vancouver the valley out enjoying the lake and waterslides.
Try your hardest not to be the one who has to do the parking, it can be a nightmare, and uncomfortable too if you don't have airconditioning.
As soon as you get parking there is enough room for everyong and you will have a great time.
If you want swimming Cultas is you best bet. On the way there is also the Vedder river, and if you follow that chilliwack Lake. Though Chilliwack lake is another hour and it's cold!