Decontaminating water tanks
This won''t help with your present problem but it''s a good way to avoid a repeat.
When I have the boat hauled for the winter, I remove the speedlog transducer from its thru-hull mount in the bilge and leave the opening unplugged. This way, if water does find its way aboard, it can accumulate only until the level reaches the open thru-hull which is considerably below the cabin sole. Any excess drains harmlessly.
I imagine it also helps a bit with ventilation via the fresh air that is admitted.
In my case (Catalina 30), there is enough slack cable on the transducer that I can place it in one of the galley sinks where it will be in plain sight as a reminder to replace it before spring launch.