AUGRH !!! That one sucks !! Looks to be like a surftec epoxy/softop with the white epoxy belly, never the less to fix it good it needs to be striped down compleetly dried out (week or more) new foam epoxed in and a new plug hole routered , new plug installed, glassed,sand coat,painted polish, done. If it were me I would have all plugs routed out, board compleatly dried and Future Fin boxes installed or the new FCS Fusion box/plug style installed and all the neccicary glass work needed.
There is no quick fix that will work and if you get water in it the foam used for epoxy sucks tons of water in it. I've seen where one crack has sucked in allmost 1 gallon of water from one use and had to go through vacume bag to remove the water. not an easy or cheap fix, but is fixable.
The cheapest fix I can think of would be. 1st make sure the board is compleetly dry air dry will take at least 2 weeks in warm/hot weather,, once dry get some epoxy resins, Q-cell or micro ballons, a new fin plug, 4 oz cloth 1'x1', white spraypaint,tape,razors etc. clean out hole and sand down entire area around plug hole approx 6" around nothing less, mix one part of epoxy resin and Q-cell till if makes a paste- using your center fin and new plug, place new plug on fin and tighten screw make sure the existing plug is ready to hold the outher half of the fin in place- now add the outher part of epoxy resin ( Hoby shops have a good 50/50 mix type for boats) fill hole and coat plug, insert plug/fin combo in hole and outher fin slot lock it down pack area full and take off exccess. Let dry for a full 24hrs then remove fin sand down smooth and cloth/resin over it ,,, anouther 24 hrs and sand smooth and re-resin if needed 24hrs again sand smooth wet sand and paint,,, done. That's about the cheapest and best way you could fix it with a minimal amount of toolage but still fixed corectly. good luck.