My theory is that Sabien intentionally tried to sink the ship over the diver's grave to deter anyone from investigating or thinking too much on it. No one's really going to question if a ship sinks in a place where ships are known to sink. The only question is why he decided to sink the ship in the first place. I don't think he's a follower of Uk'otoa or he would have taken Vandren's sphere. So maybe he's a follower of one of the other two gods and was trying to gain their favor by taking out Uk'otoa's chosen? Or it's all a red herring and Sabien was up to something else entirely.