Tammy Spencer did a double take when she read the address on her paper and looked at the house in front of her. Spencer, a volunteer with the nonprofit Enroll America, was spending a hot and humid Saturday morning knocking on doors in Boca Raton, a mostly posh South Florida city, looking for people without health coverage. She wanted to let them know about new online insurance marketplaces that open for enrollment Oct. 1. The spacious house was on the Intracoastal Waterway, complete with its own boat dock. The woman who answered the door said she already had coverage through Medicare, and Spencer went on to the next house.