As long as the heavy plate is used below the water line it will add "ballast" and help stabilize the vessel. It is the weight above the waterline that will cause instability. However, the heavier the vessel, the more power needed to propel it through the water. If I remember correctly, if one doubles the weight, the power has to be increased exponentially.
Are you considering a flat bottom boat? Characteristics: rough rider, shallow draft, slow moving, hard to maneuver.
I'm sure you've given this a lot of thought and I'm sure you know what characteristics you want. Good luck. I had a friend that built several fishing boats that were up to 120 feet in length. He would build one, sell it and build a bigger one. He was on the last boat build when he died in a car accident.
Al