I think you are a little generous with your BWL and DWL, but we both got the same results.
Somewhere between 1 and 3 pounds, so lets call it two. All at water line or lower.
A cubic inch of steel weighs 4.5 oz. so a seven inch long by 1" square bar weighs two pounds. I would have guessed more, but so be it. Or a ten inch long 1" diameter bar.
Now as far as pennies are concerned, a current one weighs 2.5 grams. About twelve pennies to the ounce, one hundred ninety two to the pound. Four of those bank roll ups get you to two pounds.
Philo, it's not ugly if its underwater. only the fish know.
Your Thermopylae would actually be a really good candidate for a pond floater. There is a lot of depth there in the hull, plenty of room for experimentation.
And MOST importantly, no open gun ports.