So, I was bored and I saw one about the meaning behind why Pitch dreamt of butterflies, and I know this has been a heavily contemplated subject over why Jack saw dolphins when he encountered Sandy's dreamsand, so I decided to google it. And this is what I found.
Dreaming of dolphins apparently symbolizes spiritual guidance and emotional navigation, which makes sense for Jack. MiM gave him the spiritual guidance he needed to realize his rightful place as a Guardian and his purpose in life. His determination for his memories and the ups and downs Jack encounters during the movie reflect the emotional navigation Jack went through in the course of the movie, such as the betrayal he deals with when he "ruins" Easter to get his memories back and the toll Sandy's death takes on his emotions.
PS: Psstt. Psst. Hey guess what. This is my 100th digital fanart deviation :3 I feel honored it's a JF one...of course, aren't they all? :3