River cruise boats, like Ocean ships and most hotels, supply hair dryers. As these appliances draw more electricity than a curling iron, that suggests that the issue some River Cruise lines have with curling irons is not simply a matter with the circuitry.
If they are concerned with the quality and features of the curling irons, the simple answer is to issue company supplied ones. Another would be a to have an on board check to make guest owned ones meet converter/ adapter standards.
On a cruise of 130-160 people, say half are women and probably half again would have curling iron. I think that would be doable to maintain safety.
i have traveled on Avalon and they allow curling irons but remind passengers of the safety rules. The stateroom attendant also keep an eye to make sure everyone is safe. They also SUPPLY curling irons at the desk to those who request one. This seems a very simple solution to the the issue. Obviously they have spotted a market need and addressed it. Several other lines have as well. This is what is good about River Cruiseing, there is something for everyone.