As soon as you clear the reef you are in Dolphin country. Look for any debris and the one with the birds are a plus. From Marathon to the edge of the reef it is roughly 4 to 5 miles. I've caught dolphin inside the reef in 60ft of water so it all depends
The biggest dolphin are typically caught (depending on the weather) in April and May. Lots of big fish in close out to about 12 miles from shore. In July there are lots of smaller fish in close but the bigger fish are in cooler water and tend to move further out to find it. In July I don't usually look hard for fish until at least 15 miles from shore.