There's no wood in our boats, so rot should not be an issue. mildew maybe...
If there's enough water to get the rear carpet wet, i'd say you're right to find the cause...
I'm sure you've opened up everything while in the water and checked the throughhull points? speedo, shaft log, rear deckmounts, Rudder assembly(someone here recently had a leak there), water intake hoses, etc... I'll bet it is the speedo. Seems like you will be able to see water coming in since there is a lot...
Now I really want to know what it is........
