Benjamin Moore Spar Varnish is typically used for waterproofing.  You have to make a rack to hang the electrodes vertical and dip them.  Then clean off the tips so they will strike.  Otherwise, they are the same electrodes normally used in air.  Here's a link to good info:

http://www.supsalv.org/pdf/cut_weld.pdf

Most commercially available electrodes have a beeswax coating, there's one that has a pvc coating.  Then there are those companies that dip their rods in their own blend of chemicals.  Now the flux composition, that depends on whose electrode that you're talking about.  Commercially available rods are generally a topside electrode, some companies have their rod manufactured with some different ingredients.  The main concern with underwater welding is hydrogen entrapment.

Charles Welch

Fellow AWS'ers
     Read D3.6 is a start point. In wet welding when I messed with market available electrodes about thirty years ago they were 6013's with polyester coating and the betters one ESAB 7014's with epoxy. Underwater welding is something I have five patents in, been recognized as a world-class expert in and I have worked in the management in the company that had the world’s largest diving contract for the world’s largest petroleum producing company and am a trained marine diver engineer CWI. There is other product besides mine. Be careful of there test data it is ideal conditions with little to do with making a real world underwater weld.  I hope this is helpful.
                        Sincerely
                               Len Andersen BSChE CWI
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