I would not recommend washing a canopy. The fabric is usually stabilized, but the reinforcing tapes generally are not. If you wash the canopy you run the very real risk of putting it seriously out of trim. I have, on occassion, wiped my canopies off with a damp wash cloth, but be careful not to soak it or thoroughly wet it. Then wipe it dry with a soft, absorbant cloth. Do not let a canopy dry in the sun. The UV radiation will damage it in a short period of time, weakening the fabric. If your canopy gets wet swooping the pond, get it out fast and dry it with towels and a fan out of the sun.
While I do technically concur with Alan, I can tell you that I have washed several canopies with no bad consequences. Washing the LINES of your canopy has much more effect on trim than washing canopy fabric.
If your canopy is just a bit dirty, then a wet sponge should definitely do the trick. The key here is to not let the thing dry in the sun. My Cobalt 75 got three "baths" at the Para Performance Pro Cup and had to be shaken out, wiped off, then dried in the sun a bit so I could pack it up and make the next round. Not the hot ticket for sure, but luckilly it didn't get way out of trim. I am definitely going to measure my lines before this weekend.