Dave, there are a bunch o' books that could get you through this process, although you could make a pirough in a couple days with info from the web.

I made one for waters just like you are talking about. I found some 9oz glass real cheap and made a real heavy canoe,75#. Unfortunately some drunk college kids dragged it across concrete and wore through four layers of 9oz to the wood in the ends. If you truly are sliding over oyster shells or subjecting the ends of the boat to lots of abuse then I reccommend using more than one layer of the standard 6oz cloth on the bottom. The thing is there is a big difference between sliding over oster beds in a floating boat and dragging the end of a boat over shells and concrete,or getting into a boat at waters edge with all of your weight over one resistant rock.

If this is a boat that you want to keep light then make sure the high wear areas get maximum protection first. That could be something as simple as a 1/4"metal strip over the ends,and an extra layer of 6oz on the bottom.

So to answer your question: yes,extra fiberglass gives you a much,much tougher surface. You don't say what kind of boat that you are making. I made a clc 16 with polyester cloth(low commpressive strength/moderate tensile strength but good resistance to abrasion) over fiberglass cloth. Although that kayak weighs about 6 lbs or so over I'm glad I did it as it is now down in the conditions you describe. I've also had the chance to see that in a year and a half that the polyester wears a lot longer than fiberglass.

Any way , two layers glass would do it. Get a book. Any book.