For water blocks any 3D program, including the $9.99 jewel case specials should be fine. There are several free programs that would work too.
Expensive programs are meant for industrial use and have extended rendering capabilities and far more complexity than you need (they all have a steep initial learning curve).
Free Programs include POV-Ray, Blender, 3Dmate (?), trueSpace1.0
A few consumer priced programs include trueSpace3.1, and a few others on the tip of my tongue
If you want to go into a hobby, eventually you'll want to check out Rhino, Hash Animation Master, or trueSpace5.2. All of them have demos.
Pros use Lightwave, 3D Max, Maya, and others.
There are tons of programs out there, and a few really good ones I simply can't remember the names of offhand. Cinema 4D is supposed to be good but I don't know much about it. Personally I don't like trueSpace but I'm trapped in the upgrade path...
But a waterblock... anything will do that.