I had a place do the above to me once. I got them the document the credit card surcharge. Two days later I called VISA and filed a complaint as part of the merchant agreement is that the merchant will NOT pass the card charges directly to the consumer**. Later I faxed VISA my receipt showing the surcharge and it was credited back to me.
**The merchant can offer a "cash discount". So if the price advertised is $1000 the merchant can charge you $975 for cash but the can not charge you $1025 once you present your card.
Thanks for the info! I know I have seen other liveaboards/dive resorts offer cash discounts, but I'm almost positive our paperwork showed there would be a 7% surcharge for using a credit card for incidentals once on island.