If you know the specific identity and relevant information of this young man, I suggest you contact the local police or rescue agency as soon as possible and provide any information that may be useful, such as name, ID number, drowning location, etc. They will be able to further investigate and confirm the situation and provide necessary help and support.
Besides, if you know this young man, I suggest you also contact his family, inform them of this situation and provide any possible help and support.