Sandy, it would be good to hear from multiple people regarding weather to/from Capri but we left Naples on a high-speed ferry to Capri some years ago in pouring rain without an issue. It thankfully stopped right before we reached the dock but was very wet almost the whole way. As I understand it, the cancellation issue usually isn't simply because of rain but high winds combined with heavy rain and whatnot that can create dangerously choppy seas. A strong electrical storm might also cause a cancellation. I'd look for more complete forecasts for a better picture as you get closer to Wed.?
The people who live on the island rely on water transport for business/errands they need to conduct on the mainland so I don't think they cancel the ferries unless absolutely necessary. It was more common to see that from Positano's dock than Sorrento's or Capri's.