Yep, changing the shutter speed does indeed let you change the colour of the background water. The faster the speed, the less light gets to the sensor and the darker the background. The slower the shutter speed, the more light gets to the sensor and the lighter the background. So you can to a fair degree control the "blueness" of the background to suit the image you are trying to create.
As Wolfgang sets out, using the LR Tint and Temp sliders changes the whole image WB unless yuu confine the adjustment to areas isolated through a Mask(s).
Masks and WB can be tricky compared to using a Mask to, for example, remove a distracting element. But if its only the background and nothing which is a prime element of the subject is excluded, a Mask can do this quite well I find.
I used to use diffusers with the Inon Z240 strobes. But now with the Retras, I just can't see any real difference with or without diffusers.