Keep it simple with soft plastic colors. A good article was just posted about this on ***
Clear water and sunny: Watermelon, watermelon red, watermelon candy, pumpkin
Dirty water and sunny: Junebug, black/red glitter, black/blue glitter
Clear water and cloudy: Green pumpkin, green pumpkin blue, green pumpkin purple
Dirty water and cloudy: Black, black and blue, dark blue.