I just killed this guy for giggles, and there was a message that my faction with 'Seru Race' got worse. I double checked it to make sure I was not imagining this. If it matters, he was swimming in the lake at the time, and had been there for ? thirty minutes at least when I killed him. Interestingly the arrows struck home when I was out of the water, and he was in the water.
Centurion Peran IS part of a quest. He is part of the "Zelniak Meat" quest. Peran sends you on the second part of the quest, which leads you to Maidens 's Eye searching for the Vampires and the powder they drop.
I think he says "Want to go for a swim" as part of the story behind the "Zelniak Meat" quest. Apparently, he is the lazy guard that is supposed to be doing other duties, but decided to swim instead. Bring him the note from Sanctus Seru and he will tell you what he is supposed to be doing.