Hi
The move from fresh to marine can be a challenge... but don't panic, we've all been there.
No a skimmer is not necessary. It's one of many different types of filtration that can be used in a marine aquarium. There are lots of successful tanks out there that don't run skimmers. I, however, am a big fan. Of all of the filtration types, a skimmer is the only method that actually removes bad stuff from the water, without manual intervention. Socks, pads, carbon, whatever other type of filter you might consider, they all TRAP bad stuff, but it stays in the tank, until you remove it (change filters, media, water changes, whatever). A skimmer pulls bad stuff, and keeps it in a cup, completely out of the system.
In addition, they do a tremendous job of helping keep O2 levels up in the system. Mixing water and a lot of very fine air bubbles does a lot for gas exchange. I've seen overcrowded systems that have crashed, bad, when the skimmer went down. Not because it lost filtration, but because it became oxygen depleted.
I use both, a skimmer, and a refugeum. They serve two different purposes. One is a mechanical filter, which uses surface tension to remove many forms of contaminates from the system. The other (if you're talking about a macro algae ref... there are other types) is a natural nutrient sink, using water born nutrients to build algae that can then be removed. If I had to pick one, I'd go with a skimmer... why? Nutrient control can be achieved in several ways, all quite effective. I like what skimmers do for an aquarium, and I don't know of another device that does the same job.
A sump is also not required... but I wouldn't have a marine aquarium without one. Why? Just makes things easier, cleaner. Properly set up, a sump system is not terribly difficult to maintain, and is quite safe. It's the best way to hide all that life support equipment that can detract from your enjoyment of the tank.
Have fun, and take your time!
