Hydro is not constantly submerged unless you are using DWC. Most others flood to keep roots/medium damp.
all hydro systems are different and you can use all sorts of mediums.
choose a system...ebb and flow, ebb and grow, deep water culture, aeroponics
choose a medium...rockwool, coco, hydroton.
I'd suggest getting a small tray (4'x4') and putting a light (400w sodium minimum, 600/1000 better) and using 1-2 gallon mesh bottom pots filled with hydroton, add as many pots as your medical marijuana states laws allow. This is called EBB and FLOW. You would flood these about once for 15 min every about every 2-4 hours. If you use rockwool, you would flood less often. Change the reservoir about once every week. This is probably the easiest system to maintain for a beginner. Do a lot of reading before you attempt to set up.....ventilation, ph, nutrient ppm, as well as specific info to the system you are attempting to set up. An easy hydro nutrient mix is called Lucas formula, GH brand nutes at 8ml micro/16ml bloom per gallon the whole way from start to finish. Feed 1/4 strength to start and work up to full.