That’s a tough one because it goes under the rubber leading edge cuff. I don’t know if you can peal that back to get access to the composite blade underneath or not, but any good repair would remove the entire crack including the part you can’t see. However, you can always experiment to see how well something works.
The first thing I’d do is stop drill the crack. Then I’d take a 90 degree high speed hand held grinder with a 1 inch 80 grit sanding wheel to scarf-sand the crack down to a knife edge on both sides. Try to slope it about ½ inch per layer of fiberglass if there is enough room for that. Like I said if there is a way to remove the leading edge cuff, make the repair and then glue it back on again later the repair will be much more likely to last.
Once you have sanded away the damage, you need to lay-up fiberglass to replace the missing material. Taper the layers to match your scarf sanded slope. Try to find a glass weave that looks like what was used in the original panel and lay it up using the same warp alignment if you can tell what that was. Usually you can see evidence of the alignment in the different layers where you sanded through them. Since it broke before, I’d put down a couple of extra layers beyond what it originally had. If you are not repairing the hidden part of the crack, I’d consider doubling the number of layers for the first 2 inches of the crack and then taper it down from there along the length of the crack.
To make it strong and flexible you need the right ratio of resin to fiberglass. By weight, you don’t want to have more resin in the repair than you have fiberglass. Too much resin will make the repair brittle. The best way to not have too much resin is to bleed it off. Look into techniques for vacuum bagging composites for details. On something like this you can achieve similar results without the vacuum bagging by laying up the bleeder layers and then clamp pressure plates over the repair until the resin cures. The alternative is to just do a wet layup. Weigh out you resin so that you don’t apply too much and try to work out all of the bubbles as best you can with a paint brush. It won’t be as strong and flexible, but like I said, you can try it to see how it goes.
If this sounds like a lot of work to you, it probably is and your time may be better spent working extra shifts to afford a new monofin. However, if you know what I’m talking about and have access the tools needed, it really is not that bad of a job. With all the necessary equipment at hand, this should be a job you could do in about 3 or 4 hours. If that is not the case, you could spend a lot of time and money just getting the tools and supplies together only to make a repair that might not last.