How to render and composite Fluid simulation?

Hi everyone.

Is there any way I can make obstacels used in Fluid simulation to be Alpha transparent, and adjust overall transparency of this fluid simulation?

(sorry for the low-res)

I used a Flip Fluid add-on and made a big wave coming towards the camera.
But I found the obstacle beneath the simulated fluid leaves a white mark, even though every object was set to be Holdout in their material node.
You can see some obstacles which are not beneath the fluid works well with Alpha transparency.

Also, the overall fluid on the front is just too transparent! I’ve set it to the default water (ocean) material in Flip Fluid add-on. I baked several times to fix this, but it’s not really working.

I’m working in Cycles, and I’ll post the whole blend file (2.81+Flip Fluid).

Are there any suggestions I can follow?

Thank you so much!