Book nothing ahead of time, book nothing through a hotel or bird dog or agency of any kind.
Do some walking and talking on the dock when the boats come for the day-ask the guys who've chartered as to how the boat was run, cleanliness, efficiency, value for the money, food/drink/whatever.
Then book only with the Capitan, if you want something specific like C&R arrange it then and again the next AM before leaving the dock.
August means there's not much around offshore except Barracudas and they'll try to kill anything they can.
If you're at all interested it's a good time of year for fishing the flats/mangrove swamps north of town in the National Park for Bonefish and small Tarpon-a quick web search will give a number of options.
Fishing Cancun in August?
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