There are several charter boats in Flamingo. It is generally calm in June so boat size isn't as important as other times of the year. It is the rainy season so the boats are not heavily booked. I wouldn't sweat booking anything in advance. Once you get to Flamingo go down to the marina; there's only one, and eyeball the fleet. There can be a huge difference between the boats. Some are questionable at best, and some are decent. I'm sure you can get a good deal on a decent boat.