You're correct that there are 4 maritime districts. After the rules note that they are divided by the dotted lines from the 3 lighthouses (which is correct), there is a diagram demonstrating movement, then an italicized paragraph that explains that the reefs (the rocky area east of the isle of fire) and the canals between the islands cannot be crossed by sea movement. So that forms 4 maritime districts defined by 3 dotted lines that can be crossed by sea and one reef area that cannot.
And, just since you didn't mention it, note that you can only go by sea movement to and from island districts that have ships shown on the board next to them.
Another question regarding Sea Movement: Does the term "island districts that have ships shown on the board next to them" mean any district with a dock and a ship/boat next to them"? For clarification, for the Isle of Wind (purple roofs, bird symbol), this interpretation would mean that only 3 of the 5 Districts on the Isle of Wind would allow for Sea Movement (i.e., the three with docks and valued at 4, 3 and 3 Points).