This review is only for the water park - we did not visit any other part of the shopping plaza.
We had a great time here over the weekend. Loads of slides ranging from toddler-size to absolutely terrifying drops. You can work up your courage as you try slides of increasing height and speed. All the slides and attractions were open and as we went here on a Saturday shortly after it opened at 9 a.m., before the crowds descended, we did not have long lines to contend with.
In addition to its big slides, the park has a play area with shallow water, fountains, lots of spraying water, equipment to climb on and small slides that would probably be appropriate for kids who are old enough to walk. There's also a "river" that winds around where you can grab a tube and just float along.
Safety appeared to be excellent. Every pool/splashdown area had at least one lifeguard holding a flotation device. The wave pool had several pairs of eyes on it. Staff communicated well with us as we got on the slides and were careful about making sure the chutes were clear before the next person got on. Helmets and lifejackets were provided; however, it appeared they ran out of helmets when things got busy. Signs are in Thai and English and clearly explained size limits and how to go down each ride (on a tube, with/without a helmet, leaning back with arms crossed etc.).
There is a unisex locker room and mens/womens toilets, showers and changing areas with hair dryers. There are several dining options, with Thai and Western food available along with snacks like popcorn and ice cream. Food is not ridiculously overpriced by Western theme-park standards, though you may blanch at paying 55 baht for chicken with sticky rice. There are a few swim shops offering hats, swimsuits and t-shirts. You can pay extra for a wristband that you can scan at photo booths around the park and have your pics go up on Facebook, or something. We didn't do that.
There are a few cons: It is expensive and does engage in the incredibly annoying practice of dual pricing, where Westerners have to pay more than Thais to get in. For a Western family of three, it cost us around 2,500 baht. As others have noted, admission prices are often changed in response to competition, so you may end up paying less or more.
The ground in the park became slippery as it got wetter with more people using it - I had to take care not to fall a few times. Be careful walking in puddles!
As at Black Mountain, you can't bring in outside food and drink and someone will glance in your bag at the front gate to make sure you're not. I suppose a really enterprising family could smuggle some in.
The wave pool, while nice and big, is underwhelming compared to Black Mountain Water Park, where the waves are huge and awesome.
There's a nice link here that describes the park in a bit more detail: http://www.huahintoday.com/thailand_news/santorini-fantastic-water-fantasy/
We happily spent about five hours here! Bring lots of waterproof sunblock, be sure to reapply periodically and wear swim shirts or t-shirts and swim trunks to protect yourself from the sun and the odd scrape going down the slides. Also bring your own towels!