It is hard to predict the weather, lately we have had beautiful day in the 70's like Sunday, then cold and rain now.
I usually answer the question to pack the suits, as they don't take up to much room, and most pools and Hot Tubs are heated year round in Southern California Lodging, so you don't see too many people sun bathing (but it does happen), but more folks on the lounge chairs in "clothes" relaxing to stay warm.
But the Water Park is a bit different, as the water in those features are not heated. (Exception would be a slide, which usually ends up in a heated pool). So how much more is the Hotel with a Waterpark?