IMHO switching from 1/2" to 19/32" ply (.59 more or less) won't make any significant difference. The difference in the quality of plywood is more important than a small difference in thickness. Plywood folks may be smart, but they need to make a profit, and they make their product as cheap as they can get away with for a given application. The 4-ply stuff is designed for underlayment or wall sheathing where it is nailed down every couple of inches, and will often warp just sitting there. I'd be reluctant to use it for a boat.
The Work Skiff doesn't have a lot of sharp curves, and I bet the 5/8 would bend just fine, although with a bit more force. The bottom is doubled, right? Here's a picture. I like that boat a lot; a genuine no-BS design with no yachty pretensions at all.
"For a successful technology, reality must take precedence over public relations,
for nature cannot be fooled."