Water will not draw anything out by osmosis – it will, in fact, be drawn in, as water is the only thing that osmoses. Saline is better, as it will draw excess fluid out of the wound, and I’ve always preferred to rinse my own wounds with higher-than-normal saline or epsom salt.
As for answers above, most contact lens solutions are sterile preserved – they used to use thimerosal, but various other preservatives have mostly replaced it. Preservatives can be irritating to already-damaged tissue.
I’m kinda guessing that a purulent boil, while not at tremendous risk for infection, is already putting the immune system through its paces. If it’s available and not ruinously expensive, I’d stick with the sterile alternative.
Not a doctor, just a chemist with a biology degree.