So, what extras do you get for your money?
I've listed this in brief already, but the actual hexagons you get are:
12 Water hexes
2 Gold field hexes
3 Desert, 1 Farmland, 1 Forest, 1 Pasture, 2 Mountain, 2 Hill hexes

Of these, the only new one is the Gold field. Should it's number come up, adjacent settlements receive a resource of their choice! We like to add this even to scenarios that don't ask for it

The main extra that Seafarers adds are the ships. These act like roads in Settlers, allowing settlers to reach the islands by travelling across the water hexagons, and are placed in the same way as roads, along the edge between two tiles.
Ships are built by using a wood and wool resource (wood for the hull and wool for the sails).
Ships represent sea lanes. They cannot be connected direct to roads, you first build a settlement at the intersection point. A chain of ships of the same colour form a shipping lane connecting settlements.
The main difference between ships and roads is that ships can be moved, unless a shipping lane has been formed.

Also introduced is the Pirate. This is a black ship. When a 7 is rolled, players can either move the robber or the pirate. The pirate is placed in a water hex and steals one resource from any player who has ships adjacent to that hex. Soldier Development Cards can move the pirate as well as the robber.

The 34 page rulebook actually only has 6 and a bit pages of new rules. The bulk of the rulebook is taken up with 11 Scenarios. These scenarios show various layouts of the tiles, using hexagons from Settlers and also each scenario has its own set of victory conditions.

For example, the first scenario is called 'New Shores' and is for 3 or 4 players. A diagram shows you how to setup the tiles, which has one main island of 19 hexes and 4 smaller islands one ocean hex off shore. Players start on the main island and have to acquire 13 victory points to win. This pretty much means the players have to explore the smaller islands to get the 13 victory points.

Our favourite scenario is Scenario 9 'A New World'. In this you literally just build the frame using the edge tiles and then all 46 hex tiles for the scenario are shuffled and placed in the grid giving a completely different game each time. You need just 12 Victory points to win, too.

SUMMARY:
So is it worth having? I would say, yes it is. If you enjoy the Settlers game, then Seafarers is a worthwhile addition. In fact, since getting Seafarers, we have not played the Settlers game without it.

Review by Brian