Maybe you use some kind of Debugview.
if i disable my debugView then there is no white Line.
I've got another question.
Did you port the waveController in 3.5 or just use an older version.
Currently i work on a Pirategame with Water. The BuoyancyController works but in 3.5 i can't find a waveController.
i really could need Liquid like u have.
Maybe my Solution helps you with you're problem.
Static Water but no whiteline.
Vector2 WaterCentre = new Vector2(0, 0);
float WaterWidth = 100;
float WaterHeight = 5;
Water = new Rectangle((int)WaterCentre.X, (int)WaterCentre.Y, (int)WaterWidth, (int)WaterHeight);
container = new AABB(WaterCentre, WaterWidth, WaterHeight);
buoyancy = new BuoyancyController(container, 2, 2, 1, world.Gravity);