Petite Martinique is reached by boat from Carriacou, usually from Hillsborough or Windward depending on operator and conditions. The crossing is short, but boats are small and schedules can change with weather, demand, and local errands.
Ask locally before planning a tight same-day connection back to Grenada. If you are coming only for the day, confirm your return boat before wandering off; there are few backup options once the regular traffic has finished.