Open water means not pools (not schools of fish). Unlike pools, open water has no special visual indication or mouseover text, and never runs out of fish. So in Un'Goro Crater open water can be found (among other places) at the hot springs. You will find pools there too, but if the bobber lands away from a pool, you'll be fishing open water.
Not all fishable liquid is water, so you'll find some references to "open lava", as on the mountain in the middle of Un'Goro Crater.
Inland means "not coastal". Generally water that is not sea, found in the interior of zones. I'd call them freshwater (and saltwater), but WoW does not obviously implement salinity**. The term can get vague, especially around estuaries, and especially in Northrend, where only coastal water that takes a specific local name (other than the zone name) is actually filled with coastal fish.
Does that help?
(** The position of character models should be at slightly different heights in the water, but to my knowledge all float the same.)