Cremated ashes in Lake Geneva?

Scattering into waters is allowed in Switzerland but not in France. And since Lake Geneva is partly in France, you have to be careful where on the lake you scatter the ashes. Also, you have to check with the boat operator if you are allowed to scatter it from a regular boat service, some might have an issue with it. There is a special company in Lausanne for scattering ashes. Google for Société de sauvetage d’Ouchy

You can do a Google search, but scattering or interment of ashes is generally permitted. But there is documentation and certain steps that are required, by the funeral home, airline, and possibly Swiss Customs.

While I don’t know about Swiss regulations, the funeral home that helped me when my mom passed away gave me “travel papers” for her, as I’m planning to sprinkle her in Northern Ireland next year (she loved travel!). Not sure if this a TSA thing, or a “just in case you need documentation” safety net, but it might be something to look into.

You can bring the urn to Switzerland without declaration or paper work. Here is the contact information if you want to reconfirm with the responsible authorities. https://www.bazg.admin.ch/bazg/en/home/information-individuals/travel-and-purchases--allowances-and-duty-free-limit/importation-into-switzerland.html#contact

I suggest you inform the airline. Some airlines appreciate if you inform them upfront. Having worked at the airport I remember having sometimes read comments on the crew's passenger list mentioning "urn in carry on" for some passengers.