Cremation in Hindu religion is two part process, 1. Burning the dead and 2. Scattering the ashes. Hindus as a part of the ritual called Antim Sanskar burn the dead bodies near a water source, mostly river banks or lakes. They burn the body first and then take the ashes to scatter to nearby holy places. Ganga river is regarded as holiest place where soul of the dead can get Moksha or escape from cycle of birth and death. While people living nearby Ganga river do the cremation (burning + scattering of ashes) on Ganga riverbanks itself, people who live far away from Ganga simply burn the body on the nearest riverbank. Some who have means to take the ashes to Ganga do it, but rest of the majority scatter the ashes to holy river in their region. Cauvery, Brahmaputra, Godavari are also holy rivers and scattering of ashes is done on these rivers as well.
Regarding the DailyMail article which states more than 100 bodies are found in water, this is unexpected. Hindus, be it wealthy or poor, follow the same two-step ritual. Antim Sanskar in poor families is organised on small scale with minimal costs involved. Hindus do not dump the dead body directly into the water. There have been some cases of tribes in northern areas like Bihar dumping their dead directly in river but that is outdated now.
With reference to water burial, article on voactiv which says
For the vast majority of the poor, however, their bodies are floated
into the Ganges
The National Geographic article has given the estimate
Estimates say 100,000 bodies of various cremation levels are tossed
into the Ganges each year.
claims made by both the articles are skeptical to major extent, at least the term 'vast majority'. Both the articles do not provide any source to verify the claim. I am highly doubtful about claim made by National Geographic as news reports of bodies being found in Ganga river are very rare (except the incident reported by DailyMail). Water burial is illegal in India. I haven't read any article in my past which claimed that bodies are dumped in river due to no money for burial, not at least in Maharashtra where I live. I assume same conditions will follow for Ganga river areas as well. People in extreme poverty might dump their dead bodies directly in Ganga but that would be due to poverty and not because of any water burial ritual.
So answering whether millions cremate their dead relatives in Ganga river, yes few million scatter ashes in Ganga, but water burial in Ganga river, not many.