The good news is you're protected by the law. Under the Affordable Care Act, insurers can only vary your rate based on your age, location, and whether you smoke. They can't take into account prior health conditions or biomarkers, including novel biomarkers measured on wearables.
Every major Republican "repeal & replace" bill has preserved this part of the law, called "rating factor limitations", although sometimes they vary the constants. For example, the Paul Ryan plan let insurers charge the elderly 5x more than the young, compared to the current law of 3x, but otherwise preserved the rule that insurers can only vary rate based on age, location, and whether you smoke.
What insurers can do is offer you a 30% (maximum) premium rebate for taking healthy actions, like going for a run, enrolling in a diabetes prevention program, or getting your blood pressure checked. But they can't tie the rebate to the outcome of the test (health status), only your participation.
This is a good point, though, and shows a case where regulation helps technology have a positive impact. If people fear their insurance premiums will go up, they'll avoid all forms of diagnosis, not just Apple Watch.