How long do plan on keeping the boat? You may want to check your other thru hulls and any screws for securing your transducer. Once the water gets in it never really dries out. It will rot the wood core. Good news is if its only a small area you probably wont ever notice. Bad news is if it is a large area it will need to be repaired. I learned the hard way and came very close to dropping a new outboard in the middle of Barnegat Inlet when my transom cracked a few years ago. There are a few ways of checking how bad it may have gotten. You can get surveyor to take a moisture reading and give you an idea or you can drill a hole(only thru the gelcoat into the coring) a foot below the suspected problem area. If water comes out the hole you may have a problem. If its dry hit all holes with 5200 and problem solved. When my glass guy suggested I drill a hole I thought he was nuts,then he pointed out that I had already drilled 6 holes to mount my transducer. If the repair is an area smaller that the whole transom its not that big of a job for a good glass guy. They just cut the gel coat back a little bit larger than the rot, dig out the mushey wood and replace with new coring and re gel coat. I do not claim to be a knowlogeable glass guy......I am however a guy that has paid a knowlegable glass guy lots of money over the years
. Please feel free to pm or email me any questions and I can bounce them off him. Good luck and when in doubt 5200 the sh*t out it.