Please help punctuate these sentences.

The first sentence is correct. The second sentence should be
"...strength of 20 women, and..."
He bought milk, bread, and cookies. In a series of items, ( or actions, etc ) there is a comma after the next to the last item.
Making it two sentences would also work.
If you have the option, I think this sounds better. :
"She jumped right in, grew the strength of 20 women, and felt right at home in the ocean."