I doubt that any guides will jump at your generous offer. If they tell you where to fish and you prove successful, then you'll ask them to guide for you? I don't often fish in SP, but the last few times I have the fishing guides I used were booked well in advance. To fish the reef you'll need more than tackle, you'll need a boat and the right weather conditions. For outside the reef you'll want to be on a larger boat. It's not fun sitting in a little boat with 2 or 3 ft swells. If you fish the flats your looking primarily bonefish although I have caught a few nice size snapper off some of the piers in SP. That was a few years ago, not sure whether many of the piers allow fishing off them now. The owners would rather have a few fish around that every one can snorkel and see, than a few tourist full bellies. Not sure how good the fishing is on the backside of Ambergris in the lagoon. My guess would be not very since most fishing guides don't fish that area. Good luck with your fishing. As Capt. Jeff says hire a local guide and they'll put you onto some fish and the price is a lot cheaper than in the States.