Alan

Is actually not as drastic as some may think. You don't have to cancel the fishing trip, or rush back to base. There are a few options. I forgot it once about 20 yrs ago. And didn't notice til water came thru the floor while I was fishing. I was about 8k from home. Lifting the bilge cover confirmed the problem. A swim wasn't much of an option - the water would have been around 12 deg. So it would be head home  staright after. May as well head home while still dry under that option. I first tried to tighten it using the tangs on the inside. They weren't strong enough to rotate the bung. But the water ingress rate was manageable - the water pressure pushes the bung in to place. So it leaks, but it is not a gaping hole. The bilge pump could deal with it easily - every 20 mins or so I ran the pump. It would pay to run the motor too - you don't want a flat battery at the end of fishing - no bilge pump and no engine. Now the problem just got a lot worse. If your bilge pump isn't working, your problem just got worse too. 

I always carry a spare bung in the boat - so at worst I could sacrifice the other one by pushing it out, and screwing the spare one in from the inside.

If you are still stuck, getting the boat on the plane will drain the hull. In my case the other day that option wasn't available due to the rpm limitation on the new motor - less than 1 hr run time.

So a swim was chosen as the option. 

But there is no need to panic, and a spare bung is a useful thing to have in the boat.

Alan