If you're planning to buy a used boat, you should do a visual inspection of all parts of the boat. You should first do a general inspection of the exterior, looking for cracks and any signs of repair in the fiberglass.
You can use sunlight to verify that the sides of the boat are straight and all one color, ensuring that the boat hasn't needed body repair. Next, you should inspect the rub rails, which run around the entire outside between the hull and deck. If properly maintained, the rub rails should be free of dents and gouges with no separation of hull or deck, or leakage under the rails. You should rethink purchasing a boat with these kinds of problems because separation is very difficult to repair.
A visual inspection of the boat's bottom should show no scraping or gouges. In addition, the fiberglass on the bottom needs to be free of bubbles or blisters; again, these are very difficult to repair. On the engine, look for any damage in the drive and prop areas. The part where the engine connects to the boat needs to be free of leaks, wear or cracking. Finally, the engine area inspection must include the transom. A transom with signs of rotting often indicates a serious problem that can render the used boat worthless.
The most important place to inspect on a boat's interior is the floor because replacing a boat's floor can cost more than the total value of a used boat. The floor should be flat without any soft areas.