Water to terrain fit has always been a bit weird.
Why not use the “Fill” tool to generate your water?
(Sorry, not Fill but Sea Level as @Khronikus states)
Sadly, terrain is just hard to work with, but here’s a few tips that might help:
Don’t use that plugin. Use Roblox’s built-in sea-level tool.
Try making the sand a little steeper. Water is already pretty hard to work with, especially when it’s on a slightly tilted surface like that. It creates weird shadows.
Use rock. Water will always make the terrain surrounding it looks chunky and weird. Using materials that match that description, such as rock, might work better.