Muffler Disconnects - Harmful?

[mudski]

I've done a bit of a cheeky today and gone and disconnected the pipe from the cat just under the passenger seat on my TB42e.
I like the sound, and that's all I've done it for, but I'm wondering if I leave it like that will I regret it.

Am I harming my power or fuel efficiency? No, maybe
Will it suck up water? highly possible if you are submerged and you stall the motor
Are the fumes gonna do any damage?Only to your lungs.

Its also illegal, AFAIK, all exhaust exit points must be behind the rear wheels. Or something like that.