Last month I stopped in to the lobby of every nearby motel in Islamorada. Here's my favorites (friendly and helpful front desk and decent prices):

Pelican Cove, www.pcove.com, 800-445-4690, MM 84.5 oceanside. options for large and small families/groups. $150/night and up.

Coconut Cove, www.coconutcove.net, 305-664-0123, mm85 oceanside. again options from small to large. $155-$195/night.