Scrub it gently with a detergent and warm water mix or a cheap car interior upholstery cleaner then hose it to rinse. It's a paintball kit bag, designed to be dragged across fields and car parks, through airport terminals etc. a little bit of water won't do it any harm at all. Just make sure it dries out really well, stuff it with newspaper once you're finished, towel off the excess from the outside and leave it near a good hot radiator.
failing that, throw it in the bath.