I’ve always loved the symbolism in the opening of KH1. The water was a metaphor for the darkness that swallowed Riku, waves carrying him to other worlds, and you’ll note he doesn’t fight the current like Sora does - much like he gave in willingly to the darkness in his heart. Sora was always the one fighting the darkness to rescue his friends. Interestingly enough, we’re never allowed to see much of anywhere else on Destiny Islands, and the only way in and out of the place was sailing through the water, much like the darkness was what allowed them to start traveling between new worlds in the first place.