Even if the watch must deactivate the app when a user lowers there wrist (though preferrably not) we desparately need for apps to be guaranteed to become active again when the wrist is raised again. An opt-in would be the perfect option. Obviously not all apps should do this so you'd want a way for users to decide.
I develop a fitness timer app that users check regularly. Once tired and sweaty if the user raises their wrist and the app is no longer there it's too much to ask for them to use the fiddly app selector interface to get the app back. At that point they probably just assume the app has crashed and will no longer use it. And who can blame them.
The only option is to select that the last activity always becomes active, but who wants their watch to do that? If this has not been addressed yet, it absolutely needs to be.
I'm currently in the ludicrious position of telling the users that are asking for a watch app that the Apple watch is no good for timing apps!