I wouldn't bother with leak testing right now. I would drain off the bad fuel and clean the tank if you can. Brim the tank off and have a good sniff around. If it's leaking then you'd smell the petrol vapours.
I would guess that water got in the tank by contaminated fuel, though the filler cap seal or my guess would be the tank breather. I am always real carefull when washing the boat down around my breather and fuel fill fittings.
Looks Slow but is Fast
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