I still have a whistle when solo diving, when you have boats pulling up beside your dive flag fishing and a safety sausage [SMB] is beacon to have a look at, a brass whistle [not plastic, but carry one as well] does the job.A whistle.
It used to have value when I buddy dived with my son. Now, as a true solo diver, it is nothing more than a complication and an entanglement 'hazard'. Dunsel gear to me.
I am sure some found their boat licences in a Corn Flake packet.