Used Saltwater Fishing Tackle

My 16" Puffer sail boat that I am fixing apparently has gravel, dirt, or somthing else inside the hull. there are 2 wells on each side, which may have let dirt inside it, but I'm not sure how to get it out. I have already flooded the wells but the water won't empty out the back of the boat, making it a real issue if it takes on water. Do I need to cut the hull open to take out whatever is inside the hull? If so, would a knife cut it, or would I need a jig saw to get through the fiberglass? If I do need to cut it open, how can I repair it?

Sorry for the major overload of questions, but I want to be completly sure that I'm safe when I take this out on the water.

Best,
Addison
I realize that there are plugs. I have pulled all of them and the water still isn't comming out because of the dirt inside the hull. I need to know how to get the dirt out of the hull, consitering it won't come out of the plugs.