Finally got around to using some old corks I found at a flea market last year.
Tools: Hacksaw, scapel (safety razor blades work too), sand paper 200 grit, latex gloves.
First I prime with 2 coats of exterior gloss white and let set for a day.
I used markers and then blended colors for the desired effect.
(Blending is achieved by quickly rubbing with a tissue that is slightly dampened with denatured alcohol.
If you are unhappy with the first results a clean alcohol dampened tissue can completely remove all marker.)
For the final top coat I used solarez, the thin version.
Lastly, hit with a UV light for a few minutes, then put in the sun to fully cure for a non tacky finish.
(A thin application of long curing epoxies also work well for top coat.)
The first one floats half submerged, and the second one is 2/3 submerged.
I'll let the fish decide which is better