The Tips in detail

 "Water means rendertime will increase significantly."

If you do renderings that handle with water in any way it will consume its time. Water is one of the most difficult textures and objects, as it is relfective and refractive in the same time and furthermore has its own texture-properties beyond reflection and refraction, such as color, waves and gloss. Refer to my "Coast", "Seafloor" and "Water Scene" images to get an impression. If you do renderings of submerged scenes do not forget that there are small particles (ie. plankton) in the water that you can see, although they are very small.

If you are using 3DSMax and want to drop objects into liquids, you might want to try the Splash! plugin. If you are out for flowing liquids, you might want to use RealFlow or FractalFlow. Using these plugins will make your animations take longer, since the liquids are calculated along some physical constraints, but on the other hand you will get rid of the problems you might know from designing the water-object itself.