Unless you want to spend pretty big bucks I would go with flash lights over most bike lights with separate batteries. (I use both.)

My own experience suggests that you can end up in situations where some moisture gets to the battery or light - no matter what you try.

The trouble using lights in wet conditions is whether they seem waterproof - its that moisture eventually gets inside and corrodes contacts and switches. The light may seem OK - but quits a month or two later.......