@RedKetchup I was just struggling with that as well

. I've found a way that works though

.
The trick is to adapt the mesh to make a turn, but to leave the UV channel straight. I'll show what I did:
I first separated my plane into several strokes (Here I did a few, but in the end it turned out that I needed much more. I subdivided them later). Make sure that the UV channel looks exactly the same (we're going to leave it that way).

How to add water to your meshes (advanced)
Then I moved all the vertices on the left side of the plane to the same position, at the bottom of the square.

How to add water to your meshes (advanced)
After that I rotated the vertices on the right side 45 degrees, and moved and scaled them so that they form the long side of the triangle and that there's a 90 degrees angle in the lower left corner.

How to add water to your meshes (advanced)
Then I moved the vertices of the long side around to make it look like a nice quarter circle.

How to add water to your meshes (advanced)
Here I tested it, and realised that it was not detailed enough; I could see clear edges in the water. So I used the option subdivide (I guess 3dsMax has a similar function?) to cut the triangles into multiple smaller ones. I had to increase the amount of triangles by a factor 3 (subdivide 2 times), resulting in a total of 27. In Blender this option automatically updates the UV map while still leaving it straight.

How to add water to your meshes (advanced)
Then this is the result


How to add water to your meshes (advanced)
Let me know if it works!