When are good times/circumstances to build harbor settlements in Settlers of Catan?
There are 2 types of Habour (or port) in the game, and they have different best times.


The 3:1 ports are always handy, more so in the late game. However, there are few reasons to have one early.
The 2:1 ports are worth building if, and only if, you have an excess of production in a particular resource.  This may just develop late game (especially with brick and wood once you've got no expansion), or you may purposefully give yourself an excess with initial placement.  This is most common with sheep.


However, rarely is it worth starting on a harbour unless it has excellent numbers (5-8, for example).  In general, you should be able to build on a 3-resource hex and have an uncontested build to the harbour for your first settlement, which should give you more resources early on before the trade would be useful.