The back side of the screws are, or should be threaded inserts. Open it up, then as you try to remove the screw look at the backside. If they're spinning you'll have to secure them somehow to get the screw out. A little bit of waterproof grease or anti-seize on the threads when you put it back together should keep that from happening again.
Carefully using a small amount of adhesive on the threaded inserts prior to putting the screw back in should prevent that from happening again in the future also.
You could also upgrade it to a piano hinge that is hidden inside the gap that helps spread out the forces trying to pull them out. Also if placed correctly, you can get the hinges out of site for a cleaner look.
Here is a image search link so you can see them and how long of one you may need: Boat Piano Hinges
I contacted Merritt Marine and they are sending me the materials. They are a great outfit and responds right away. I’ll try the powder technique and see how it goes. 13 years and just now the screws are loose. Hope for another 13.