And yet, Serge's death by drowning could also fit the phrase "has returned to the sea". This double meaning is what makes scripts like CC good.
As for the whole Wazuki kills Serge thing, I was kind of hoping for some in-game evidence to support it. The way I see it, FATE wants to release the Prometheus lock on the Frozen Flame, and if Serge did not need to be alive to do that, CC would have been a much shorter and less confusing game.