There's a few problems with this.
1. Phone is ringing, so you have to time your key word in between the rings, that's if your ring tone even have any breaks in between.
2. You're in the shower, all the water noise is going to interfere with the phone's ability to listen
3. You still have to put your phone up to your wet head
4. You'll request for a feature that activates speaker phone with a spoken word to get around problem #4, but no app exist for that today. (Imagine if that feature exists. The person you're talking to could potentially activate speakerphone for you, or worse, other feature that are implemented)
5. Unless you take 2h long bubble bath, 3 times a day, is this really a problem? And if you do take 2h bubble baths, you shouldn't have to stress over answering every call.