It's as hard as you make it.
You would have to top off with water once a day to keep the salinity right, but you could go for just a live rock tank, leave it in natural light and watch all the hitch hiker life that comes in (like sponges and smaller invertebrates). this would be easy, far from taxing and certainl surprise newcomers to marine at just how much tiny life is in a tank.
A hermit or two should be ok, but putting a fish in would add quite a high bioload and probably cause problems. Live Rock would only cost about £10-15 on a small tank and then you just need a powerhead and some substrate.
You could probably go some low light corals with a small desk lamp.
So many people view the SW side of the hobby as harder or more taxing, but I spend less time on maintenance on (and more time watching) my SW tanks than my FW.