Just getting started: what is it with metric motor shaft diameters vs. english diameter propellers?

I am in the early stages of planning out a simple, 3-motor ROV. I’m am thinking of using some 800kv (-ish) brushless outrunner motors with 50/55mm propellers, like these.

The problem I am running into is that while brushless motors seem to always have metric diameter shafts (3mm, 4mm, 5mm),. most of the boat props seem to have inside diameters measured in fractional inches (like 3/16"). Do I care? I’d be mounting the props with a compression-style prop adapter.

My prior experience is with quadcopters with propellers that bolt directly to the motor casing, so I haven’t had to worry much about this in the past.