While looking for answers to this claim, I found none that supported it to be true. All sources stated that drinking large amounts of water cannot cure COVID-19. Although sources did voice for hydration as a factor in staying healthy in day to day life.
For my sources I used:
The World Health Organization (WHO): https://www.who.int/southeastasia/outbreaks-and-emergencies/covid-19/What-can-we-do-to-keep-safe/fact-or-fiction
WHO has a whole page on COVID Q&As. One of the questions was if drinking lots of water has the ability to cure COVID. The WHO states that though drinking water is important for health, "There is no evidence that drinking lots of water flushes out the new coronavirus." As a source The WHO is likely not biased as it is a medical research group.
BBC: https://www.bbc.com/future/article/20200319-covid-19-will-drinking-water-keep-you-safe-from-coronavirus
It's in the title of this article from BBC "No, drinking water doesn't kill coronavirus." This article explores and debunks the online 2020 rumors that drinking water every 15 minutes will help cure and prevent COVID. These rumors were circulating in 2020, when there was an abundance of online rumors about COVID "cures" and "facts". Most of these false rumors stem from personal fears/coping with the perceived lack of personal control that the pandemic caused. There is no scientific evidence to support the idea that drinking water will flush the virus into your stomach acid where it will die. BBC as a source is likely not biased in related to this topic, as many COVID "curing hacks" came from Americans on the republican/conservative side (such as Trump supporters) and the BBC is a British news source.
and PubMed: https://pmc.ncbi.nlm.nih.gov/articles/PMC9510225/
This PubMed article briefly explores the 4 cases of Hyponatremia caused by excessive drinking of water. 3 of these cases actually over hydrated themselves because of the belief that large quantities of water would cure their COVID. PubMed as a source most likely does not have many biases as it mainly just uploads studies and medical information. However, a case group of only 4 people is not particularly impressive or prove much.
While there is evidence that drinking water can help with overall health and your immune system, it is not a magic cure-all. Overall, most viruses are transmitted through droplets in the air, or left on surfaces that are then touched and transferred. So washing a few extra germs into your stomach would not keep you safe from sickness.