Help for my outboard motor

I am asking for any advice of what may be wrong with my outboard motor. It is an older model 6 hp Johnson. It always starts and usually runs like a top. I use it for duck hunting in flooded timber so you can imagine the trash it encounters. The flooded timber is pretty thick so I just putt through it at quarter or half throttle.

The problem started a couple of years ago. Occasionally the motor will make a strange noise and white smoke will pour out of it. I can't describe the noise. It is not a hissing or a clattering or a grinding noise The smoke does not have a burning smell so it actually may be steam.

The water pump is working just fine. After several seconds the noise and smoke quits. It happens so randomly. It may not do it while motoring out but may do it a few times while motoring back. Temperature does not seem to be a factor since it will do it whether it is cold or warm.

Any suggestions by the OBers? Thanks in advance.

ETA: I forgot to mention that the motor shut down twice during these episodes. The first time was a couple of years ago and the second time was earlier this season. It was like the motor was seized since I could not pull on the pull rope. After some time the pull rope loosened up so that I could start it again. I had the water pump replaced a few weeks ago just to be sure water was pumping at its maximum and the mechanic said the compression on the cylinders was fine.