Sea ports don't produce troops, so unless your immediate neighbor has an attack route by sea, ignore them until your military is strong enough to start naval invasions. The optimal thing to do is to take a water route to the enemy capital with at least 2 large units, generally the AI only sends one or more units to defend it. And for the love of god, don't ever EVER leave the route to your capital unguarded.
As tempting as it is to grab an island capital for that extra layer of protection, always go for one surrounded by cities, so you can build up your military quickly. When signing a peace treaty, be sure to do it with a distant team, and preferably the one that seems to be winning.
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