Robin has pretty much summed up the basics.
I'll add that you've got to be careful of aluminium fumes, and the main benefit of a water table is dust control. Plasma dust is a akin to saw dust. It gets everywhere!

I've only got a non-CNC plasma cutter, and cut quality is very much dependant on consistency of all the variables, which with CNC you can control quite tightly unlike by hand.

DaveK, or Stirling (at least I think it's Stirling who makes torch height boards...) are probably the best to comment if they're around, as they've got a lot more experience.

A CNC plasma table is on my list of wants, but there are plenty more projects ahead of it.