Hi Robert, at 1 : 24 scale the barges work well without altering them so not sure why you are doing a generic one. They could be sailing with water up to the deck line or empty so the water level of the hull in the water is incidental.
I know the designer of Celia Jane and he is very precise and you could do a lot worse than following his plan. I have seen one sailing and they sail very well.
Most of the models sail with a weighted keel underneath maybe 6 to 8 pounds in weight and about 8 inches in depth.
You need the surface area of the keel to stop drifting sideways. The barge boards work at full size but do not have much effect at this scale.
The sails will have been balanced for Celia Jane so if you change the dimensions you are on your own with a sail plan.
Generally the jib and the main sail go out in parallel. The mizzen sail less so but you may need to experiment here.
Locate the hull exit points for the sheets beneath the point at which you attach the sheet to the boom. The topsail you might want to make removable depending on wind strength, but otherwise it follows the mainsail.
The weight is going to come out at up to 20 pounds so you can build 'heavy'. In simple terms you need a bow and a stern section and these are joined together by side planks and the bottom plank.
The difficult part is making the rudder look right, if you look on the Internet you will see it was a very complex piece of equipment using left and right hand threaded traversers to work the rudder.
There is loads of detail about the deck detail and winch on the net. Simulating canvas can be done with covering a wood shape with ladies stockings material using varnish to keep it in place and then trimming it off.
I was on a Thames Barge for the day 7 months ago it was the once famous Will Everard, the family did not allow the name Everard to be sold with the barge so she is just known as Will. I have some full size sailing experience and was allowed to pilot the barge under power for half an hour or so. There was very little wind that day.
Picture is of my daughter driving and the other is they opened Tower Bridge for us to go through.
Good luck and to be honest it is just as easy to make a scale model as to make a 'generic' one.
Good luck with the build
Roy