Sorry for all the noob questions...but that's what noobs do, ask questions.
I have had great difficulty trying to plop the maxis seaport. So much so, I've only been able to do it 1 time and that was in a place I didn't even want it. I was just excited it plopped. The red goes on land and the blue goes in the water, I get that much in theory, but in practice I just don't seem to be able to do it. I've tried terraforming the shore and even used PEG's CDK Leveler. Thanks in advance.
I don't know if this will help, but from what I understand the seaport needs to have a shore line that's relatively flat and the you have to have a plateau type thing both under water and on land where the seaport will be placed. I usually have occasional problems plopping the seaport, but in the end I usually figure it out.
Perhaps you might want to post some screenshots of the area where you want to build your seaport. I think that would make it easier for people to help you solve your problem.
Sometimes when using PEGs leveler lots to create an harbour area, after demolishing these lots the terrain kind of "remebers" that something has been built here. That's a known bug, and there's a thread somewhere about thet. In that case it's a good advice to dezone that area with the normal dezoning tool. Possibly this is your problem ..
Apart from that - yes, if you could post a screenshot, that would be helpful for us.