The closest US Embassy to Dominica is located in Bridgetown, Barbados.
I highly recommend you register with this Embassy. In the event there is any kind of emergency you will be accounted for here on the island and your family (or whoever you designate) will be able to receive updates about you.
American Citizens Services (ACS) Unit of the U.S. Embassy’s Consular Section is located at Wildey Business Park, Wildey, St. Michael, Barbados; Telephone (246)227-4000; ACS unit fax (246)431-0179; e-mail : BridgetownACS@state.gov.
Canadian citizens can register online at http://travel.gc.ca/travelling/registration. To update your profile or add dependents, send an email to firstname.lastname@example.org.