he says that a fisherman found him at a ship wreckage with a rune at his hands and then the dragonborn says him that if he finds anything he will tell him. So my question

Unfortunately, no. I wish Bethesda had made a quest though.

That would have been nice. It's good we got something extra with Sapphire though with Dragonborn.

I'm still annoyed that I don't know what the rune actually looks like.

The TG has some good NPCs, I think Beth missed the mark by not expanding on some of them.