Water Body Ocean at least, by default will recognise landscapes and be culled underneath it, and the waves will attenuate down to O at the shoreline; in Early Access however these features dont seem to work properly with large worlds using World Partition. They do work on single landscapes though.
You also may need to adjust its Blend Mode in the details panel to get the correct behaviour with an imported landscape.
Regarding underwater post processing - make sure you open up the collision settings in the details panel of the Water Body Ocean and extend its bounds in X and Y far enough to be present wherever you are in the map. By default they are fairly small values.