>Welds are quite strong -- it just extends the metal. This is especially true when the baseline quality of the metal is not high.

This is not the case at all. A weld almost always weakens the base material. And you don't just use whatever steel is the cheapest to build a ship. You use what is appropriate to the use case. There are cheaper and more expensive options within that category, but you make it sound like you can just grab whatever is cheapest in the yard that day.

There's so much that goes into material selection and handling that this comment confidently hand waves away.