You will have a great time leaving from this spot by boat or kayak. Try to pick a day when the winds are on the calmer side to really see more of the fish and rays Underwater. When you boat on the slow side in the three foot depth range, it is much easier see the nurse sharks down below. The ramp is free, 24 hours a day, and typically is not filled up with boat trailers.
Special is the whole zone on the southern side of the Seven Mile Bridge. Take a good look at the water navigation maps and be impressed with all the islands out in the backcountry. The bridge is right there if you want to use the boat ramp to get out oceanside.
4000 Overseas Highway
Big Pine Key FL 33043
Related: middle keys boat ramps