The general rule of thumb is to NOT go on the weekend. As for what day of the week, I'm not sure there is enough data to know for sure. Probably Wednesday.
More important, I think, is time of day. This time if year, it is hot all day. I would go at opening (9am) and if you can, be done by a little after lunch time. Like the regular theme parks, the water parks get busy in the afternoon. At least that has what I have read from the Unofficial Guide, and what I have seen going to Typhoon Lagoon this last late June.