Any paint or hardware store carries polyurethane. Nothing special. I have also used Marine Spar Varnish. Both work at stopping the discoloration, but they both darken the spot where you paint the board a bit. Not a perfect match in color.
I have also used it on some small dings where the wood was clearly damaged (2mm deep) just to seal any cracks. Nothing that penetrated through all layers of the board, but just a precaution to prevent the wood from absorbing the water.
It takes about a minute to dab on the paint with a small brush, dry overnight and on the water the next day. One coat seems to work fine.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.