I have a 2006 331.
The 331 water tank sensor is located towards the aft end of the tank. If you remove the floor panel above the tank access hatch and look rearwardin the gap below the floor, but above the tank you should see the sensior wiring.
The sensor in my 331 occasionally stops working and to date I;ve managed to get it back by reaching intio the tank and ‘sloshing’ the water towards the float at the rear of the tank. I assume the float sticks. Give it a try !!??
I’m not sure how to access the sensor from above, but suspect it either needs a part of the galley floor to be lifted. There is probably a screwed down section in the floor either in the space under the lower step at the rear of the galley or under the dinette porthand rear seat, (not sure if ‘Miss Nauti’ is the live-aboard layout, if it is not, then I think there is a cupboard to port of the step).
Hope this is helpful, good luck.