Hmm, have you noticed that when Kylo interrogated Rey with the force he said: "You Imagine an ocean. I see it. I see the island." That makes me to think of 3 theories, considering the end of the movie where Luke was on an "ocean planet" with "islands":
1) When Rey was a child, barely able to remember much of the surroundings, she got a glimpse of the two most noticeable things - the ocean all around her and the Island. Maybe she was born at this planet raised for some years until Luke decided to hide her, so Snoke to not be able to sense her presence and give away Luke's position. Luke as the only real and left alive jedi should be strong enough to cover his presence, similar to what Sidious did all these years but a child, force sensitive one, might not be able to understand what to do nor how to do it and could potentially expose them. Also, possibly Rey was seen as a child by Kylo and that is why he was so shocked at the beginning when've been told that a certain "girl" got the droid and left the planet. Maybe he knew that Luke had a daughter and was heading his way. Kylo in the movie is 29 years old, while Rey is 21 (Movie is 30 years after battle of Endor where, Kylo was born 1 year after it, while Rey - 8 years after the battle(info from wookipedia)). So, when Rey was 3 years old (from the vision), Kylo was already 11.
2) Rey was in the new academy where Luke trained his new padawans - Kylo yet again saw her and new Luke had a daughter. After the purge he was probably searching for her but Luke - afraid to lose his only child - sent her away, similar to how they've done to him back then, sending him to Tatooine. From the vision the "hand" that grasped the young Rey and the voice, were the same as the alien that gave her rations to eat...maybe he was a trusted someone to keep an eye on her, similar to how Anakin was with Watto (another junk trading master). Either way, Rey might have seen the planet which had ocean and island where the temple was located BUT - that planet would not be the same one from the end of the movie but similar to it cuz otherwise Kylo would know where it is.
3) And final theory of mine is that Rey simply had some connection to Luke via the force, who was observing her in a new way (telepathically), so he shown her the planet he was located - the beautiful ocean and the island. Maybe that was her dream where she really would like to be and this vision gave her strength to continue to live until the day comes when her parents will take her back to this place where she would like to spend with them.
(At the end of the movie, if Rey really was strong in the light side - if she asks the right question the force will respond accordingly - so if she was asking herself "Is Luke my father" the force would give her visions of that to be true, similar to how Luke thought about his sister, without knowing her and was like "who might that be?" - second later he said "Leia, she is my sister" and Kenobi nodes that to be true. Would be epic if movie ended with "Father?" instead of silence but JJ is know to make mysteries and if that was necessary one - so be it.)
What do you think about it? Which island and ocean was Rey dreaming when being "young"?