Dive watches are robust tools that have historically been used out of the water just as much as in it. Take for example the mountaineer Uemura and his 6105, or many of the other Seiko-sponsored climbing teams. In cold environments like that, leather can be a good choice.

Or say you want to wear your favorite diver to the office. Leather is a good choice there, and it can serve as a reminder of your aquatic pursuits. Or as a reminder of any sort of adventure.

Just because leather is a bad choice for your use case doesn't make it bad for everyone.