Has anyone come across this sort of thing before please? Seems to be some sort of swimmers or, from the depiction, divers watch.

It dates from the very early thirties and has an unusual call. 35,5 s-t1 movement in relatively dire condition. The hair spring is a bit tangled so the staff doesn't move and the dial Is pretty disastrous though it seems to be some common size. I can't find any reference online to Omega making watches with cases like this

Personally I have my doubts as to whether it is truly waterproof in the modern sense despite being a divers watch.

I hope my title isn't in any way misleading

 

Broad jump on a windy day?

 

The relief pattern was likely a reference to a competition for which the watch was awarded.

 

Yes, a windy jumping broad.