The feeling invoked by the author is to show the freedom of accepting of fate by the narrator.
Step-by-step explanation:
The feelings described here are numerous in small amount of time. The feeling of the narrator is at first described to be strangely fearless.
This makes one think that there is something to fear which is why it has become fearless.
But then the author describes the feeling by the use of imagery and we understand that Phil is in fact drowning and these emotions are his understanding that he will now not survive at all.
There is no chance to survive so he accepts his fate and feels strangely free.