Cancelation fee

We paid a deposit for the special they were running, that included a boat ride and lunch that costs R499. Then they said it's a R1000 for a couple, that was still fine with us. With the rainy weather, the river is full so the boat rides are cancelled. They will give us the R80 back, and we can come have lunch. When we cancel they take R15% on the deposit. They did not say anything about the river being full while they were asking for the deposit few days back. It's been raining for two weeks. They should have stated that as a possibility while asking for the deposit. We cannot postpone as our guests will be traveling from the free State this weekend. Isn't this covered by their insurance? Why should we be charged for this?