I'll trun the question around - why would you want to take them into water?
Nearly all Panerai are supplied with two straps - a leather (alligator/croco etc) and a rubber one for 'wet' wear...
The leather Panerai straps are treated to 'resist' water, and sweat etc., and you can certainly wash your hands without ruining one - but then with such an easy strap changing system, it's simple to whack on the rubber one if you're off swimming.
I know you've all seen this shot, but I like it...and it's relevant.
With respect to the choice to supply a rubber strap with the Luminors and not (typically) with the Radiomirs, historically speaking I find this odd. The Radiomir was originally designed as a divers watch...however nowadays it seems to have evolved into a 'dress' watch, leaving the Luminors as the rugged do-everything model.