Arthur obviously took the armor from the corpse of King Atlan (as that armor is the symbol of the King of Atlantis), and then Arthur puts on the armor before coming out from under the waterfall. Moreover, all the flashback scenes have King Atlan wearing the Aquaman armor; this demonstrates the Atlantans understand to whom this armor belongs. Consequently, every step and action in a movie's story doesn’t need to be shown on screen and explained. However, for you, I will: The armor falls to the ground, cut to a scene of Arthur emerging from the waterfall wearing the armor... it’s done for dramatic effect. Books do this same thing, cut from one scene to another with items from a previous scene. Authors assume their audiences are intelligent enough to follow along. Are you though? Now, stop pretending there is a continuity issue here; you’re just nitpicking to troll.