Polyester resin does not have as high a bond strength as Epoxy, and is not as flexible, but is significantly cheaper and easier to work woth, other than the excessive eye watering lingering fumes.
Proper surface prep is extremely important, and contaminated bilge water feeding a long crack that has been there a while with lots of flexing going on is very much undesirable.
Before any sanding, scrubbing with acetone with many a dollar store scrubbie sponge in one hand and a clean rag in the other to immediately absorb that which as loosened in the other, is a good starting point. Get the entire area as spotless as possible in this manner. Do not allow acetone to puddle, as it will begin to soften the resin. Pick away any loose fiberglass in the immediate area of the crack and let it breathe out and dry. Do not try and speed up this drying by the application of too much heat too fast. Keep it under 120f.
Do Not do ANY sanding, until the surface is completely free of contaminents, as sanding would merely force them into the sanding scratches impeding subsequent secondary bonding.
Once the wood near the crack is as dry as it is going to get, iuing some sort of filler like microballoons or cab o sil or fumed silica, to fill the void is required, and then rebuilding the skin over the crack should be done with feathering the strength and thickness of the repair/reinforcement in mind, as any hugely strong spot, induces a weak spot right next to it.
Another good read is the Epoxy book By System 3 resins.
https://www.systemthree.com/pages/literature
Proper mixing of epoxy cannot be stressed enough, both in ratio and thoroughness within the mixing cup. Mixing by volume can lead to errors and epoxy not upto its full strength characteristics, and removing this epoxy is an absolute nightmare.
Make sure the mixing stick can get into the corners of the cup, scrape all 5 sides of the mixing stick on the side of the cup and mix these drips back into the rest of it thoroughly, do not allow any to drip down the outside of the cup, and mix it twice as long as you think is required. Strong light will often reveal swirlies in the mixing cup that would otherwise go unnoticed and lead to an improper cure if applied. Always mix epoxy resin and hardener thoroughly and completely. there is little room for error, and fixing errors it 3x the amount of work, minimum.
Epoxy is not cheap but it is superior in every way, but for resistance to UV light and excessive heat. It is not as forgiving as polyester resins to work with, and this area is why it is often spoken poorly about, by those used to working with polyester resins.
Most epoxy has little VOCs and only a slight smell, but I can smell polyester resins a mile off.
Cured Epoxy dust is inert, but when not fully cured it is still toxic, and some can react badly to it, and some will develop a sensitivity to it and cannot be anywhere near it once this sensitivity is developed. So take precautions, and do not clean any epoxy that gets on one's skin with Acetone. Distilled white vinegar can be used to clean application tools, and skin of epoxy where as with polyester resin one has to use acetone, and human skin presents no barrier to acetone.