The game was built for the Saturn and ported to PS1 (versus being built independently for it). During that process, graphical fidelity and effects were lost and audio transcoded in a lossy manner. Sprites were lower quality, the water effects were worse, textures were downgraded, shadows were lost, etc.

This is a pretty bad demonstration since they took literally zero effort in syncing the two side-by-side til the end, but gives you an idea:

https://www.youtube.com/watch?v=UL6Z9xvt7h0

That being said, it's not so much worse that PS1 is unplayable. It's just that if you had the choice between the two, the Saturn version is slightly more pleasant to look at and listen to.