From Gizmag.com Water release seat belts

[Midwest Amphicar]

Escape Belt releases your seat belt when your car fills with water
For most of us, chances are that we will never be in a car that plunges into the water. However, if you consider yourself “at risk” for that sort of eventuality – say, if you’re a rally driver or Jason Bourne – you might be interested in the Escape Belt. It automatically releases your seat belt when exposed to water.