Larry,

"Not intended for use below the waterline" means it will not hold up if continually in the water...defined as more than 4 days constant submersion. This is per the definition of several different varnish companies I contacted a year ago.

As long as the hull is removed from the water every few days to dry out, all should be fine. The warning is intended for "docked" or anchored craft which aren't removed from the water every day or two.