Heroin when exposed to water for a long period of time can convert to morphine I believe, but this shouldn't be much of a problem unless it's been wet for a long time.
You can dry it out pretty easily there are plenty of ways: Put it on a plate under a lamp, use a hot plate, use an oven on a very low setting, use a dessicant, let it sit out in the open air until it dries, etc.
If you use heat to dry it just keep the heat low so it doesn't degrade the heroin.