Sea Pen - dry berthing without lifting

I have often thought of a similar device to kill the banacles with chemicals while still afloat. How much water pressure must there around the outdrives or sterngear when the water is pumped out.
 
I saw a another device recently - a floating sausage around a boat that contained some kind of anti foul. I was told it was a chlorine based chemical!!
 
only prob is marina berths are pretty tight. I notice they dont show other boats in the pics
 
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