I'm going to ask a really silly question, even at the risk of becoming labeled as a complete moron for eternity.
Consider a wooden boat, made of mahogany for example, with shining sides having 32 layers of the finest varnish one can find. The varnish looks great and protects the wood from water, sun and other elements. Below the waterline, however, you would find a coat of primer and an anti-fouling paint.
What would happen if you were to use varnish all over, also below the waterline? Would the bottom be swarming with barnacle? Or would the varnish prevent the bottom from breathing, thus resulting in rotten/mildewy wood over time?
The varnish would be applied only to one side of the hull (outside). Has anyone tried it and if so, what was the result?