That's completely fair. Deposits should be ensuring that the dive op has revenue WHEN operating. If they decide to cancel, it doesn't matter if no one signed up that day, they were not making any income. And I would expect (someone like @Wookie can comment) that a business would base their pricing on number of days of operation, knowing they will at times get blown out.
When in Florida, I'll be sure to look at the fine print and choose my charter accordingly.
I booked a series of dives in WPB and paid for them in advance. Weather blew out a couple of them, shop refunded without question or prompting.
I chose the dive operations based upon reading this forum and direct messaging a few of the people here who dive in WPB regularly. I wasn't disappointed.