L'Express des Îles operates fast ferry services between Dominica and Guadeloupe, Martinique, and St. Lucia several times a week. Ferries arrive at the terminal in central Roseau, which makes them convenient — you're in the middle of town as soon as you arrive. The crossing from Martinique takes roughly 75 minutes; from Guadeloupe, about two hours.
Schedules shift by season and are occasionally suspended for maintenance or adverse weather. Check current timetables at expressdesiles.com before travelling.