Amon was drowning in the ocean and suddenly woke up deep underwater, so it makes sense he would desperate use his powers to save himself, not rationally thinking about the fact that there might be witnesses.
I just started playing the finale again, and I agree elements feel very rushed. We should have been introduced to Iroh and the Navy in a standalone episode before this. They just show up out of nowhere in the previous episode, we had no mention of them before this.
Also, at the open of the episode they're hiding with this crazy guy with wild hair. Who is this guy? Again, someone they should have introduced in a previous episode.