I feel your pain Jules,
After the end of my trip to Exmouth in the Magnum, all was good. I prepped the car and lashed everything on the boat for the trip south. The boat was in good order and I had a comfy bed to sleep in on the way home. I drove a thousand km's and stopped in the dark for a sleep at about 9pm.
I descended into the cabin and stepped into an inch of cold water in the boat. WTF? AFter spending a couple hours sponging the boat out I crashed out. The water was brackish in taste, so couldn't figure where it came from. I checked the under bed bladder which still had 40-50 litres of water in it.
On arrival at home, I stripped the boat and cyphoned the water from the bladder, removed it from the boat and filled it on the dry driveway. Withing a minute, there was a spreading wet patch on the pavers. Sure enough, a pinhole in the underside of the bladder. Old age, hardening plastic and a now permanent crease had exposed a pressure point which had abraded a hole from the vibration and movement of the bladder on the way home.
Like you said, luckily, it was fresh water. The brackish taste I believe came from the salt embedded in the carpet which had been down for a couple of years.
The upshot is I now have squeaky clean bilges, washed carpet and compartments, oh, and a new bladder and filler hose.

Glad you got out of it reasonably scott free too

DM