Boaters who cruise along the River Soar in Leicestershire have said if mourners continue to sprinkle ashes in the water, the river will become unusable.
Hindu and Sikh mourners are using a boat service to sprinkle ashes on the river around Barrow.
Graham Chetwynd, from Sileby, said coconut shells and flowers thrown into the river could damage boats and ultimately the environment.
But the boat service is popular, with mourners travelling far to use it.
Mr Chetwynd added: "The place where ashes are scattered is not very deep and any amount of ash is going to cause a build-up of silt and ash.
"Eventually it will be impossible to get through there."