I visited Siam Park last summer, and I can honestly say it was the best water park I've ever been to (and I've done both of the Disney ones!). The first ride I went on was the Tower of Power, which was pretty terrifying. It's a huge, straight waterside that is horrifyingly close to vertical - sooo scary, but great to be able to say that you've done it. You do go through a tube that passes through a shark tank at the bottom, but there's water everywhere so the natural reaction is to shut your eyes, so you don't even see the sharks or know you are passing them at all. The slide itself is quite bumpy - the joints where the separate parts of the plastic slide have been put together are uneven, and scratched my back in several places on the way down, leaving it stinging badly for the rest of my visit to Siam park, and mildly for a couple of days after that.
My favourite rides in the park are Vulcano and Dragon - I can't choose between them. Volcano is great when you're inside the "volcano" part and there are all the special effects - a great addition to the ride. Dragon has some really scary drops where I was sure someone would fall out the raft, but everyone was fine (needless to say).
Kinnaree was brand new when we went, and it was amazing - it was kind of like a cross between Dragon and Vulcano in that it had drops AND a Vulcano-like dish. The snakes had quite short queues when we went, so we got on them a few times and thought they were great fun. The racers were also really good for the family and didn't have too long a wait time.
All in all, it was a great day, and Siam Lark trumps any other watermark I've been to hands down.