I think you should realize that you're penalizing the player for role-playing his character. In the future, this player is not going to take any kind of flaw that will keep him out of the action. It's possible that, with this current character, he suddenly overcomes his fear of water so that he's not left behind in the XP race.
If that's okay with you, no problems. But if you keep it up, you'll get characters who are built for one thing and one thing only: smashing heads. Anything that could keep them out of the action will be cut out of their backgrounds, or just ignored. And perhaps that's as it should be.
Hmm... there are a lot of things to consider here.
Anyway, if I was running this game I'd have made it clear what the goals of play were. If it was about getting into character, or smashing down the door, or whatever. I'd reward XP based on that criteria.