The water park I mentioned was indeed Suan Siam or Siam Park.
It is a fun FAMILY place but not cruisy at all. It is full of young children with their parents but there are hardly any teens/twens there.
We went there because my boyfriend wanted to relive some of his childhood memories and actually had a lot of fun just hanging out :-)
And Icon is right it is not exactly 'on the way' to the Big C.
But if you go there you could make the detour as i did - although not by choice.
After the water park had closed my boyfriend and I simply could not get a taxi at a good price. Those guys where asking for 2000 baht to take us back to silom.
So we took a bus that was across the street from the car park. It was one of the little green ones but I don't remember the number.
It went all the way back to silom road and it did go past the Big C and also The Mall Bangkapi which has the rooftop water park that ninja mentioned.
It took 2 1/2 hours to get back and although the bus had no a/c
it was a fun ride since I got the chance to see a lot of Bangkok that usually just passes by quickly in the taxi on the highway.
Bangkapi and Ramkamhaeng had some nice eyecandy going on and off the bus too
Oh and I think my boyfriend paid 12 baht for both of us