Re: what is it?
Although the picture is deceiving, this is on top of the pontoon but below the deck. You would have to use a mirror to see down in there once the plug is removed. I too thought it might be for some add on device but there is simply no room for much more than your hand. Also, there are 3 chambers in each pontoon and one of these on each chamber, leaving me to think that it is a pressure test plug. We'll take the plug out this weekend and see if it goes all the way down into the tank.