Why did they wait til the rains filled up the lock? I get that it helped the plot that the woman and Aquaman got to spend time together in her apt, but if they all knew his health was suffering, why not drive a little farther to the open water that was clearly shown in the movie was just past the lock?
I thought about that too. They could have just driven to the next ocean and release him.
I think, Elisa used the "rain has to fill the canal"-thing as an excuse to keep the creature/meerman a while.
It was a selfish thing to do and not based on reasoning. She felt alone and had a connection to him/it.
So she set the date when to release him as far away as her conscience allowed her to. The rain was just an excuse.
Well before they busted him out, he was on the verge of passing out due to Strickland torturing him and very injured. When they actually busted him out, the scientist told them that he needs that medicine frequently and also to maintain a 5 to 7% salinity level in the water he was going to be in.
So while yes, Elisa was keeping him because she developed feelings for him and she was being selfish, part of it was also to nurse him back to optimal health.
They needed the water level to rise enough so that he could be released into a larger body of water to go back home to South American waters. But like most people said, I'm sure they could have found another area to release him into water without needing to wait for rain.
The lock is part of the water system. They could have literally walked 1/4mi or less and dropped him into a river/ocean. Stupid plot hole indeed, like the entire fucking movie.