6 inches of water will drown you easily, if you bonk your head on the way out, landing face down.
In that boat with a dog that can move without notice, I'd even wear a jacket, once warm weather sets in you should consider the expensive kind that inflate on their own, so they aren't as cumbersome or warm to wear.
I'll admit to not wearing them too, in my Bass boat on calm waters, but it's by far more stable.