Get a router and machine out a 10mm x 2mm deep band around the daggerboard in exactly the right position. Mask up the edge of the rout and infill the exposed area with a contrasting gelcoat colour. carefully wet and dry the new gel down until flush and then polish the whole daggerboard........ April fool, forget that and just get a pen!
I also like to write abusive messages below the band so that they can be read as you fly a hull past someone - they soon wear off and you have to think of some new ones!