You may want to take a look at the ends of the deck boards and see if they interlock or not. Most composite deck boards don't (i.e. are not tongue and groove). If they're not interlocked or overlapping, you could consider removing one board and then spreading the rest of the boards back out to provide a small space between each one. This is a bit of work depending on your deck size. Your other suggestion about drilling drainage holes will also work - drill as few holes as possible, and drill them between the boards. Don't go any bigger than 1/8" to start and see how that works out.