The teardown says the Apple Watch Series 2's adhesive is "much stronger" than the original Apple Watch, and iFixit assumes this is related to improved water resistance. It also appears that Apple has added a larger metal shield next to the Digital Crown, which is also likely an improved waterproofing measure.
Apple Watch Series 2 is marketed as swimproof, with an improved water resistance rating of up to 50 meters that makes it safe to use while swimming, showering, fishing, washing hands, jogging in the rain, and similar shallow water activities. Apple does not recommend scuba diving, waterskiing, or other high-velocity water activities.
The teardown also revealed the Apple Watch's new S2 chip, larger Taptic Engine for haptic feedback, new antenna module with a GPS, the addition of a second microphone, and a redesigned speaker that is designed to fill with water, then vibrate to pump excess water from the body of the speaker.