Regarding maintenance ... At least with a large tank, my experience was that after some time, perhaps 3-4 months, the environment becomes very stable on its own. This was 80 gallons, it used mostly natural filtering with "bioballs" in a connected section at the back of the tank. It's important to have some means of moving the water around for good oxygen exchange, I used those powerhead type pumps.
The only real maintenance was regular partial water changes (mandatory as salinity increases when water evaporates), cleaning the glass, and periodic water tests. As I said mine was very stable, but that has a lot to do with the volume of water. Marine tanks are not as complicated as people tend to think, they just need a bit more attention. As long as partial water changes are done regularly there's not much to worry about.
They can be an entirely different experience from freshwater. For example, I got a bunch of this "live rock", it's basically old coral rocks that are full of life. It is very porous and many things live in the rock. In fact, almost every time I looked in the tank I saw some new life that I never saw before. One day a big crab crawled out of the rock that I didn't even know I had. I also had cleaner shrimp breed and there was hundreds of them. One thing I NEVER could get to live is anenomies. After several tries, my belief is that these shouldn't be taken out of nature, they should be left alone. They simply don't survive in aquariums long term. In fact I think there should be bans on removing anenomies from nature.