Though fans only saw a glimpse of Luke's X-wing in Star Wars: The Last Jedi, it turns out the ship's fate was developed behind-the-scenes and completely intentionally within the canon.
Mild spoilers for Star Wars: The Last Jedi follow.
The book The Art of Star Wars: The Last Jedi is loaded with details from the now-playing-in-theaters Episode VIII, including why Luke's X-wing as seen underwater on Acht-To.
"In February 2013, Doug Chiang painted Luke Skywalker's X-wing starfighter sunk beneath a tropical turquoise sea for The Force Awakens development," the book reads. "The idea would finally bear fruit in July 2014, when Kevin Jenkins returned to the concept in early pitch art. "
"'I wanted to tell a slightly different story: As Rey finds Luke on the island, maybe she stumbles into these caves and upon the X-wing, rather than seeing it from a distance,' Jenkins says," the books reads. "In 2014, he wrote, 'Rey explores the island, trying to understand Luke's self-imposed exile from the universe. She comes across a cave, sunk at high tide within the base of the island. Here she discovers Luke's scuttled T-65 X-wing, rusted and rotting away in the water. It was set alight before it sank. It shows Luke's commitment to his exile, as he destroyed his only means of leaving the planet.' In the end Rian Johnson chose designs more similar to Chiang's initial painting of the sunken starfighter just off the coast of the island."
Star Wars: The Last Jedi is now playing in theaters.