If you are looking to create realistic seas on Roblox, texture wise, it wouldn’t be possible. Essentially, there is a limit to how much quality a texture can have. Do not have the exact number, but it is less than 4k (which below it probably wouldn’t be considered realistic).
However, if you want to generate water, you’ll need to use The Perlin Noise Generation Method.
Edit: Even though the link I have showed you is old, it is definitely still relevant.
Edit 2: Here is another link on how to use perlin noise generation.
Perlin Noise to Create a Map