Congrats on your new to you boat. Welcome to the club.
You are looking at a high water mark.
At some point in time your boat was filled with water.
Maybe the former owner can share the circumstances that caused this boat experience. Maybe not. Sometimes we want to block out old bad memories.
As they say... it is your boat now. Work to affirm responsibility # 1. Keep the water outside of the boat.
Not always easy, but the mission non the same.
Looking at? That is what you do when you inspect the boat. If this is the first time, then get a good flashlight and spend some quality time with your boat looking at everything that is below that 2.5" line. Electrical... inspect for corrosion.. Wood.. inspect for dry rot. Pressed board. inspect for swelling. Take appropriate action (remove/replace, repair, leave alone) if you find things unacceptable.
Good luck!