the Help has a good description check that out. People seem to do spheres when doing water but the best thing is a poly object reduced to a single vertex.
With objects it puts a sphere around every vertex so if you want a 'soft' box you need to have lots of vertexes in the box or you just have a few spheres in a box shape or one sphere (depending on the settings).
Splines are much more powerful in a metaball than you usually see used (but maybe I'm just not looking in the right places). Once again the help has some good example pics.