In terms of the likely identity of our body, a number of saints have floating stones associated with their burial.

A strong contender must be Samson of Dol - in the Life of Illtud* his body is placed in a sarcophagus, which floats back to his Master Illtud's harbour for burial¹. Equally St Gildas requested that his body be left to float on the sea in a boat with the stone on which he used to sleep (his 11th C. Life by the Monk of Ruys [Translation]* then goes on to detail the boat sinking and the body being split between various locations, doubtless to explain the disparate relics scattered around his churches). In addition, St Teilo* (Translation of 12th C. Life) took a stone coffin so large only ten oxen could pull it, and floated it in front of his ship in order to pick up a king, Gerennius, who was waiting to die. Finally, another strong contender is St Carannog (Translation of 12th C. Life*), the father of all altar-floaters. He was given an altar, the colour of which was unfathomable, while living in a cave. This altar, floating, preceded him when he sailed across the Severn and he frequently cast it upon the sea to follow the path it dictated, occasionally loosing it with various consequences (at one point it ended up as a table in Arthur's court, that would throw off all objects placed on it).

However, with the exception of Gildas, none of the saints specifically requested their remains be lost, and with the exception of Samson, none asked to be returned to Illtud - though Gildas was also Illtud's student².

A final question is why the man wanted to be lost: it seems like people used saints' graves to swear oaths on, and this person wasn't keen on the practice. A lot of the saints were a) fighting Pagan stone-worshippers (at least according to the later Lives) b) miserable so-and-sos, so it isn't entirely surprising³.