there isn't much more equip. needed to keep sw fish than there is to keep fw fish.
you need the basics such as the proper size aquarium, a filter(undergravel, sump, canister, or power), and a flourecent light equipped with a color enhancing bulb for marine tanks. if you plan to keep coral and/or anenomies you will need a power compact flourecent light equipped with both actinic and day bulbs. its also good to have lunar moon lights. theres also metal halide lamps but theyr'e insanely expensive and the bulbs are too, but they are the closest thing to sunlight.
theese kind of lights are not required to light a fish only aquarium.
it is highly reccomended to have a protein skimmer. this device draws water from the aquarium using either a pump or an airstone and mixes water with air and churns it in a reaction chamber where proteins attatch to the microbubbles which ascend to the top and forms a foam which is pushed into a collection cup. whith out a skimmer you will have serious water quality problems and get nasty biological buildups in your aquarium and probably dead fish. and lets see, what else...... ahhh yes a hydrometer. this is used to determine speciffic gravity to determine salinity levels and you should keep a battery operated air pump and extra batteries in case of a long power outage. most of the other equipment depends on which fish you decide to buy
One other thing, if you are planning on having a reef set up (with fish, inverts like shrimp, corals etc.) it is strongly recommended to use an appropriate amount of live rock to do the filtering for you (about 1lb per gallon is often quoted).
For a filter I would be against a undergravel as this makes more problems in saltwater by making more nitrAtes and canisters i would recomend a sump(wet/dry). Or you can gor for a more natural setup and do live rock you would want 1-2 pounds per gallon. And would definitaly want to have a protien skimmer very important piece of equipment and not something to get cheap personally I don't like the airstone ones as they have to go in the tank or sump and don't make as fine as an air bubble as the venture ones do. If you are going to keep corals just softies like polyps mushrooms stuff like that then you would want power compacts. If you want triadnca clams and SPS and LPS hard corals then you will want metal halides. And the price isn't to much different as in power compacts you change about four of those bulbs for about forty bucks a piece thats 160 dollars as for the halides you only have to replace two for 80 bucks a piece thats the same they are a little bit more expensive to run and produce a lot of heat and the ballast for them arn't cheap either but well worth the money as the corals colors just come to life plus you get the water ripple effect on the aquarium floor with them which is really cool theres is a lot to saltwater and ten times that we don't even no yet but best advice i can give you is do your research be very patient.