Hi,
Yes you still need the power heads, but a return pump is to bring the water from a sump to the top tank, so you would need that as well if you have a sump, or you would empty out the display.
You want to get as much oxygen into the tank that you can. There is less oxygen in saltwater then fresh, the warmer the water the less oxygen as well, and most folks keep a tropical set-up.
Power heads spread the oxygenated water throughout the tank. The open top allows as much surface water as possible to be exposed to the fresh air, and then circulated throughout the tank by the power heads and any other means, such as a return pump which causes more surface movement, or a skimmer. I always made the surface water in my displays look like it is boiling, even now while I keep seahorses and use airlines instead of power heads, the surface is agitated enough to look like it's boiling.