First thing is find the leak. I'd see about a SealTest place that pressurizes the rig, then uses a soapy water spray and looks to see where the bubbles appear. Fix it on the outside first. It only will get worse and more damage will happen until this gets done.
Once the leak is addressed, I'd go for the Git Rot. This solves the problem the right way, although it takes patience and quite a good amount of Git Rot as the wood will soak up a lot of the epoxy. If possible, the best fix is cutting out and replacing the rotted section, but this might not be doable if the board is a load bearing item.