Sounds like there might be a couple of things going on. The fact that the Nav lights came on in the rain could be a due to water shorting out the switch, or causing a short somewhere in the wiring (bad insulation). Not sure how to find this, other than with a hose and perhaps a multimeter. This should be covered by your warranty, and I'd contact the dealer.
The dead battery could be due to this short, but is also common when the boat sits with the battery connected and the automatic bilge pump cycles. The wire to the "auto" feature is connected directly to the battery, and over time the pump will drain it. I found that this took a couple of weeks on my 170DC. After searching the forum, I found a choice of solutions:
1. Discoonect the battery when the boat will be parked for more than a couple of days.
2. Install a battery cut-off switch ala Chuck's recommendation
3. Install a switch in line with the power wire to the bilge.
After discoonecting the battery a few times, I decided to install the switch in line with the bilge pump wire. Instructions are here:
bilge switch
It was a pretty simple installation, and has worked out quite well. The only problem I've encountered is with my failing memory. The switch effectively cuts all power to the bilge pump, so if you forget to turn it on, you've disabled the pump in both auto and manual modes. I used a rocker type switch, with an indicator light to help remind me, but the wife still catches me once in a while when she asks if I've turned the bilge on. She's got a lot less faith than I do in regard to the "floatabilty" of our boats!
Ron