Been there - Done that on my '08 MS 34HT
Comodave is right on the mark w/ cause.
MY 34HT has a bulkhead that stops 4-6" below the side decks and I strongly believe the hull flex above the top of the bulkhead concentrates the stress right at that point.
What I did w/ the help of one of my yard guys (not sure who was the lead / helper but he knows fiberglass work so appreciated having him)
It was a 2 man job in my case because access aft of the bulkhead is around aft of the gen and fwd access was around the main engine so it was almost essential to have someone on both sides.
I cleaned off the sound insulation best I could - sanded and wiped w/ acetone.
Then cut FG mat various sizes so the inside reinforcing was multi layered but tapered to distribute the flex stress over a wider area. I cut a notch roughly in the bottom center of each piece so they would partially slip over the top of the bulkhead. Mixed West System epoxy resin (best choice IMHO as epoxy is a better adhesive that polyester) and we then worked together to brush some on the area to be patched and each layer of mat smoothing as we went. I forget how many payers we ended up using but I'd guess 4-5.
We ended up mixing 2 batches as the first started to set as we worked.
Easier to mix a couple small batches than rush to do it w/ one larger one.
Note above req'd a fair amount of prep to remove hoses & other stuff that was in the way - not sure what access on yours is like.
Nest step was external repair / repaint.
I tried simply gouging out the gelcoat & filling & painting but it didn't hold and a crack reappeared during the season.
I ended up grinding out a wide tapered area and building it up w/ increasing size layers of mat & epoxy resin. Final filling / fairing was done w/ conventional FG reinforced body filler - fast set & easy sanding. My guy didn't see the need to use a true marine epoxy filler above the waterline.
Paint info can be found on my Bacchus
website in the Links are
see MS Hull Paint info & Links
Color match was excellent and you can barely find the spot repair even when close up.
If you have questions or want to discuss any of this either post or PM me here or via email on my Bacchus site.
I will PM you my phone # if you want to talk live.
I did the external repair in several steps over several days including the priming & final sanding w/ wet/dry. I had a friend that does truck repair including body work help me w/ the top coat spraying as he had the equipment & skills.
I helped him prep his hull accent stripe and we did both jobs in 2 days.
I had reasonable luck spraying the primer w/ a PreVal sprayer available at any big box store. PreVal sprayer