Intriguing thought running a small compressor but I would have thought it is easier to use vacuum off the manifold, not much to go wrong with a bit of pipe. No wires, fuses, motors or anything else to go pop just when you really need it.
As for the question of air flow, the whole point is to increase the ventilation rate to remove excess water vapour from inside the dizzy cap. Like MHM said, when you dunk your engine in cold water, the outside of the dizzy cools down and condensation forms in the cap. It isn't too bad if you keep the engine running but as soon as you switch it off for a few minutes you will have the mother of all misfires.
So even if you have taken sensible precautions to protect your engine for a bit of swimming it would be wise to pop the dizzy cap off afterward, let it vent for a couple of minutes, wipe it down and give it a spray of WD or similar. In the alternative, just seal and vent the dizzy.