Just starting out with Seafarers of Catan—what are the key strategy differences between Settlers of Catan and Seafarers?
For me the big strategy for seafarers is to realize that most of the time the islands are not worth it. If it is very convenient to go for an island then do it, but usually I find that the resources I expend could have been better used elsewhere.
(Though if everyone is following this strategy, then the islands are great because of the free space. Basically you want everyone else to go for the islands but you.)
To me, the big benefit of playing Seafarers is the diversity of maps, and the ability to play random maps. I find the default random map is a little sparse on land, but this only makes you think harder on where you want to build. These new maps open up new strategies, and change the dynamic of land settlement from being contained on a single land mass. Depending on the map and the starting positions, you could want to focus on boats or completely ignore them. As said above, you might want to strike out visit new land in order to get more victory points.
Depending on what scenario your playing, and what the map looks like, changes how you play. Seafarers games take longer, but if your playing random maps, it makes each new game different.
There is also the discovery map, where you never know what tile your uncover next.
When placing 1st or 2nd in the basic game, I always look at the board and determine which of the two basic strategies (ore/hay & wood/brick) works best. If I place 3rd or 4th, and my opponents know what they are doing, I often have to use the opposite strategy (or one of the others i.e. balanced/monopoly/etc).
Seafarers adds wood/sheep as a third equally viable strategy. I enjoy this since it gives you more options when placing 3rd or 4th. Since both wood/sheep are needed for boats, players with higher probability wood/sheep hexes will be more able to explore. Don't under value the 2 bonus points for building on an island. Unlike longest road and largest army, you can't lose these points.
One key difference to the wood/sheep strategy in seafarers is that it is a two-part strategy. Building towards a high-probability outer island hex allows you to make-up for a lack of that resource on the main island. In this way, you almost shift strategies mid-game.
My first choice is to build towards a good Gold hex. You can do anything with it.
My second choice is to build towards a good hay or ore hex. Both allow you upgrade your cities and free up settlement tokens for further expansion. Good access to sheep/hay/ore also allows you to hoard development cards and go for largest army & victory point cards. (If you are using the wood/sheep strategy, I assume you already have good access to wool.)
If I don't have good access to brick on the mainland, building towards brick helps to build settlements without having to trade-in, and also to go for longest road.
You need sheeps and wood to build a ship, so sheeps are slightly more important.
Second, most games give extra victory points for the first settlement on a new island. This can be nice, but realise that you will have at least two settlements on a coast, reducing the number of goods for them.
In my experience, small islands are usually not worth going for in the beginning of the game because the amount of territory you get access to is not worth the resource requirements. Obviously, the larger the island, the closer it is to your starting position, and the more dots worth of production, then the more worth while it is to go for.
However, even if you don't settle any islands for the first 80% of your game, there are two big strategy changes:
1) Ships can be moved (1 per turn). Building many ships can be a strategic choice that offers many tactical advantages during the game:
With ships opening up so many more opportunities, wood and sheep are more valuable in Seafarers than in Settlers.
2) The bonus victory point(s) offered in many scenarios for building your first island settlement makes for an interesting end game strategy not available in basic settlers. As discussed in What is optimal end play in Settlers?, it can be hard to win Settlers by increasing one victory point at a time, as players gang up to slow the leader. In Seafarers, islands make it possible to catapult to victory with a 4-6 victory point turn. You do this by waiting to settle your first island on the turn you think you can win because you have the right combination of resource and development cards, as follows:
Building your first settlement on an island counts for 1 victory point for the settlement, and 1-2 bonus points depending on the scenario and the edition of Settlers of Catan edition you're using. Older editions have a 1 victory point bonus, while the newest edition usually offers a 2 victory point bonus.
About gold: In most games I've played, islands with gold usually take a lot of resources to reach and then attract the robber disproportionately. So going for gold is rarely a winning strategy.