There are no direct flights from Martinique to Dominica, and fortunately there happened to be a ferry (Express des Iles) that ran on the day we wanted. Booking tickets online for Express des Iles was very difficult - we finally ended up using a travel agent in Dominica (Whitchurch Travel). I called them up, and after many attempts to email them our info (their email server was blocking all hotmail and yahoo mail) we finally got 'tickets', which were just a scan of the receipt. However, we had no problems using this to get the real ticket at the port in Martinique.
The ferry was only 15 minutes late - excellent service compared to Liat (The Carribean Airline). Unfortunately the catamaran (modern, covered, with a restaurant on board) ran after dark, so we didn't get a view of the islands. The seas were extremely rough, and most of the passengers got sea sick. Even with 2 gravol/Dramamine, (motion sickness tablets), Ian barely hung on.
Arriving into Dominica wasn't exactly a smooth process - the arrival forms that the ferry company had given us were wrong, so everyone had to scramble to fill in new forms. Our guesthouse in Roseau wasn't that far, but the dark wet streets, lack of road signs, and an inaccurate map made it a little difficult to find.