The following features are relevant for improving fluid-object interaction.
Show Collision Geometry
This option helps you to fine-tune fluid-object collisions, e.g. when particles go though a surface, and visualizes how RealFlow | Cinema 4D "sees" and objects. The helper mesh is updated when you change parameters of the "Collision" or "Volume" tags. If the collision geometry is not visible reset the scene to frame 0 or scrub the timeline.
Show Collision Velocity
This option is available for animated objects and displays its velocity as red lines: the lines' length is an indicator for the object's velocity. If the collision velocity vectors are not visible reset the scene to frame 0 or scrub the timeline.