This product is an in expensive quality spar varnish packaged for BoatCraft.
The "Spar" varnish is an enamel type
varnish that keeps well and works great on small jobs 
like paddles, gunnels, rudders and dagger boards (on boats that
are dry sailed) etc.
These items need regular touch ups.

Varnishes like enamels, single part polyurethanes and two
part polyurethanes 
are only good in the water for up to two weeks before they start to 
get soft and can easily be
scratched off.
 
There is not a varnish made that will sit long term below the water line at this time.