I received the Marina, and then earlier today the Seaport and could not get the thing to build. On the map, I was using the largest sized square on the bottom left, building near the shallow water inlet on the east side. For some odd reason, I happened to come across a momentary lapse of reason, and wondered if the shallow water might have something to do with it.
Sure enough, using the 'Surface Water' tool under "Environment' dug out a underwater trench deep enough to allow the Seaport to build. I have found that having a straight down (nearly) vertical shelf next to land allows the Seaport to be built pretty far out into the sea.
I had searched into this conundrum for a couple weeks and had not found an straight answer... so hopefully this helps the Sim City Creator for Wii addiction to become even stronger... ;)