If your relative died by accidental drowning there would have been an inquest and these are usually reported in the local press. Try papers for Pontypool / Monmouthshire as he was from that area and also for Symonds Yat where the accident occured.

For holdings of Welsh and Monmouthshire papers the www.newsplancymru.info is a useful site. Newport Library and the Gwent Record Office should be able to help.