I've read this story 5 times. Thought I got it But nope I don't get it.

I think the friend, shouko, who could swin, died. If you see on page 8, she's farther into the water than emiko. The mother being understanding of emiko's trauma caused by seeing her friend drown, tells her that little girls don't have to force themselves to swim.
The last panel, saying her red soda summer days are over may allude to shouko not being able to drink strawberry soda at emiko's house anymore (shouko asks for strawberry soda in the beginning because she has melon soda at her home?)