Uncle Fred, do you have any plain drill bits on your boat?
When I have to match up the size of a bolt, I will hold up a drill bit behind it and look at the two, changing bits until I find one that is the same size to the outside of the threads. Then just take the drill bit along to the store, do the same thing in the store, and you've matched up the size of the bolt, and you can pick matching nuts from that.
You would also want to properly torque the new nuts when you install them, so you will also need a torque wrench if you don't have one.
A good hardware
store or machine shop will be able to sell you a "size finder", which is a plastic or metal sheet with holes stamped in it and bolt thread patterns cut in the side, or a set of flat guages with notched sides. Usually $5, 10 at the most, this is the "proper" way to measure odd nuts and bolts and a good investment.