If you say the water rose and deposited shells as we see them then the water would have had to reach many different heights in the flood year and stay there. That means the world wide water level would have to vary very ( ) widely across the globe.
The mountains raising does not, in any case, explain what we see. If you think it does explain, first, your understanding of the facts about the seashells and then, in some detail, how they got there.