The large boat was you afloat in the sea of life, maybe getting away from it all, populated with personal aspects of yourself, which you started to calmly kill the thoughts of, as if not knowing the reason. You had no thoughts, so it seems you were emotional, reactionary.
Parts of you put up a fight and a few parts were running.

Satisfied after that, you went to the back, more private, hidden part of you, to finish your self or your thoughts off.
You made a stand and went backwards. Interesting you let yourself fall first, instead of shooting which would then make you fall. Indicating you fell over the edge first which was like half killing yourself.
No wonder you were still alive, shooting yourself underwater, under the surface.

You tried twice more before you realized you were out of ammo - See? You were out of ammo or motive. You did not really want to kill yourself. Jumping and then pulling the trigger is almost like hanging yourself without the rope.

There were still two parts of you alive, the captain in control of yourself on this strange journey, and some other important aspect of you - maybe just your self value ~ so had no problem helping you find the means.
This part of you found the ammo hidden where you sat, and you returned to the edge but I think it's the last thing you remember because it's the last thing that happened pertaining to the meaning. What happened after probably had no bearing on the meaning or was just your after-thoughts or contemplation of doing it. But you never did it.
You never wanted to kill your main self.
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Killing these people in your dreams was justified - at least to you. Or you wouldn't have done it. These people were all unjust or guilty or bad parts of you that you wanted out the way. The reason that wasn't clear fits with why you didn't know why you were killing them - probably an emotional outburst where you were not using your thinking.

Now it's explained why after you woke up you felt really depressed for most of the day.