There are only 3 boat ramps "near" Big Lagoon 4 if your in a kayak.
1. Sherman's Cove on the navy base - I think you have to be military
2. Big Lagoon state park - costs 10 bucks unless you buy a yearly pass for 64 bucks
3. Galvez Landing - it's on the other side of a 45 min no wake zone on the way to get to Big Lagoon
4. Johnson's beach has a kayak ramp - it costs 8 bucks I think to enter the park unless you buy a pass not sure how much.
The best deal for me was Big Lagoon state park. Nice ramps, not far from prime fishing spots, and if you buy a pass for a year it pays for itself if you launch there 6 times.