That's probably because HP Water sucks.
With the HP nerf, it's is only worth using if the target is 4x weak to the HP type or if the pokemon's movepool really sucks.
Think about this, what does HP Water hit harder than say, Energy Ball?
Use HP Ground if you insist on an HP. At least that actually damages Heatran.
I have my reasons gatsby knows it:)My question is why have HP Water? You got Shadow Ball to hit Fire types, and the only Fire type that resists Shadow Ball is Houndoomand Pyroar
For Rock/Ground types you have Energy Ball, so unless you specfilicly wanted to take them out, HP water isn't worth it, it's not even listed in its sets :/ (Plus I dont have a HP Water spread on any of my Ditto)
edit: I could settle for hp rock as well if water isnt possible