Other mistake: Artistic license via CGI physics is taken for all the characters in the water to keep their hair from ever flowing over their faces, no matter how long or how the currents might affect their hairstyles. When characters emerge out of the water, their hair remains nearly unaffected.
Other mistake: When Arthur meets Atlanna on the beach, she steps towards him. When the camera zooms out, there are no footprints in the sand.
Other mistake: During the final battle between Aquaman and king Orm, notice the propellers, both left and right prop rotate in the same direction, however the physical lay-out of the propellers are not matching (mirrored), thus one prop would give forward thrust, the other will give rear thrust. Not logical with both rotating in the same direction.
Other mistake: Aquaman and the princess were in Italy, then they are on a boat, but there is no great deep trench in the Mediterranean, and they couldn't have made it to the Atlantic in that fishing boat.
Answer: You said it yourself some might have stumbled upon it, others could have used the map, which is why it was hidden in the first place.
How could they use the map when Arthur was the first to find it...quite frankly to stumble on it is impossible, you would have to randomly travel to the trench, go through the entire kingdom, see a tornado of death and decide to go in, get spat out and not eaten, get to shore, check behind a waterfall and follow it down...bit much?