I don't really see the point of the montage involving Luke spearfishing and milking the space manatee. I don't see much of a story-telling reason to see how Luke has survived or become very rugged.
It's true that the opportunity to show his starfighter submerged was taken, and this serves to (falsely) foreshadow that he is actually on Crait.
However, this could have been achieved with any shot of the water.
Do we have any commentary from production staff about the reasoning behind this scene? Any suggestion there was concern that if not depicted, fans would treat his survival as a plot hole? Concern that people would assume he regularly left Ahch-To for provisions?