aluminum hull plating pitting

‘In the commercial world it is based on % of loss from original thickness. Anything more than 25% loss in thickness and regulatory bodies will make you replace it. In the case of pitting as you describe you can measure the depth of pits and average the depth to get a loss % over a few square feet. What is the original plate thickness? If you have 3/16” deep pits that doesn’t sound good unless you have unusually thick hull plating.

Who is measuring the pit depth, plate thickness and area size?
Guessing by finger touch or real devices?
Audio or acoustic measurements are really needed here. A surveyor or tech that knows how to really use the equipment is needed also.

I have witnessed a few false alarms by overnight surveyors.
In the past on a Striker and Roamer I had worked on, Surveyors had caused false alarms when they found blems/pits in the surface fairing.
On another Roamer, we thought the fancy equipment was working and still reported bad plates. Again the strange micro balloons fairing was messing up the readings.

So, before you call out the plasma cutters and order material, really make sure you really have a problem.

You can, but it will be somewhat larger by the time you get it prepped for welding. Still much better than replacing part of the hull.

It depends if you can get the Alum clean enough to weld. When you heat alum via welding it draws any contaminants to the surface making it difficult to weld older, dirty Alum. There are some cleaning products your welder should be familiar with that can help. You will need to fill the pits with weld and then have multiple passes of weld put over the top extending well beyond the pit area in all directions - several inches anyway. This process is called clad welding.