In Settlers of Catan, when you place your initial settlements, are you allowed to place them on harbors, or can you only place them on interior spots that have 3 land hexes adjacent?
From the rules:
The starting player places a settlement on an open intersection of his choice. [From the setup instructions]
Intersections are the points where 3 hexes meet.
In order to control a harbor, you must build a settlement on a coastal intersection which borders the harbor.
Water hexes are hexes, so the points at which two land tiles and two water tile and the points at which of one land tile and two water tiles meet are intersections.
During the initial placement, you are allowed to place a settlement on any open intersection (as constrained by the proximity restrictions), so you are allowed to place on coastal intersections, so placing on a harbour is a legitimate placement for a settlement during setup.
This is supported by the official FAQ:
Q: Settlements and Cities - May I build a settlement at a harbor during the set-up phase?
A: Yes. This is not contradicted by the rules.
The upside of placing on a harbour is obtaining the harbour.
The downside of placing on a harbour is that you touch at least one fewer resources than you could by placing elsewhere. This means your potential for gaining resources will be lower, the variety of resources you can obtain will likely be lower, and you'll start with fewer resources (if the harbour was your second placement).