specific age and model of your 260
The tank is under the cabin floor from the cabin steps fwd to the base of the mast and side to side about 3-4' wide.Make sure to purge all the air out of the ballast tank, if you have any air in there it'll slosh around like a water cooler bubbling. I hope this is the issue but..........A wet spot around/under the floor board at the galley usually indicates leaky hatch boards. The cabin slats and poptop will leak pretty well in a blowing rain, wrecking havoc on the flooring. Get a canvas cabin cover to keep it dry.Better check the through hull fittings too as there was a recall on certain ones. Is there any significant water at the bilge pump under the aft bed? Lets hope not.Good luck, Michael