Hi cks. Yes, equally sorry for your issue. And hopeful your "fix" works as advertised. Personally, I'm of the "reef 'em out and replace" school for tankage leaks aboard a recreational powerboat. Knee-buckling expensive, for sure.
However, I must throw out a caution flag for part of your commentary:
To it insure I never have water in this part of the bilge again I intend on building a wooden damn to prevent water for getting back there. I’ll most likely use epoxy...
As you well know, bilges are intended to allow water in them, and to allow free passage of water throughout the deepest portion of the hull to location(s) where it can be easily and completely removed. Should the architect that designed your GB have slept during that part of class, or the builder had his head where the sun don't shine, and failed to provide this free passage THROUGHOUT THE BOAT, thus condemning your tankage to failure, then doom on him.
But not a good idea to attempt to damn off or create a "waterproof" area in some part of your bilge. First off, it just ain't gonna happen. Water will have it's way with you, as he operates in concert with Mr. Gravity, and drains towards the center of the earth. Always. And there's ALWAYS water in a boat. Not necessarily by intention, but it's always there. In your case, perhaps from simple condensation on the exterior of the tank that then migrates to your bilge.
In many, many decades of time spent in bilges of my, and other person's bilges, I've NEVER seen a "waterproof" area in a bilge. Yes, I've seen lots of dry (as in, perfectly, 100% dry) bilges. Never have I seen an un-drained void space in a bilge that didn't ultimately find itself full of water, in need of proper drainage, usually stinking to high heaven, and contributing to exactly the issue that caused your tank to fail.
And yes, there are "watertight bulkheads" on many small boats. But standard practice is to provide adequate limber holes within the bulkheaded area to allow any standing water to drain and be collected and disposed of overboard. To do otherwise is simply a bad idea. If you are trying to bulkhead-off your tank, in an attempt to prevent subsequent moisture intrusion under your tank, you're doing absolutely, unequivocally the wrong thing, and will hasten the repetition of your current problem.
It is highly recommended that you revisit your tankage, with the thought of opening access to the tank (particularly the bottom), and ventilating it properly so any water that does collect there (and it will, for sure) will either drain away, or evaporate quickly. I feel you pain, should your architect and/or builder not made that provision during the build, but I heartily suggest you make best efforts to correct their oversight. Fixing leaking tankage is bad enough the first time. To do so again is simply heartbreaking.
And a final comment, you state your tank is fabricated from 3mm 316 stainless. From what I've seen on other GBs, the water tankage is more likely fabricated from other-than 316. Probably more likely 304, and probably not passivated after welding. Other than custom builds, I've never seen an OEM 316 tank, and certainly not the preferred 316L.