To allow you to render the particle fluid, Houdini must “surface” the fluid. That is, build a polygonal surface based on the positions of the particles in the fluid.
When you use the shelf tools to set up a particle fluid network, the tools automatically set up a new Geometry object containing the result surface from the fluid simulation (by default called particle_fluid or sculpted_particle_fluid depending on the tool).
The particle_fluid object contains a node importing the particle fluid visualization, and nodes to import and convert the particles into a surface. The 
DOP Import node with the visualization has the display flag (blue circle), so it is used for the viewport. The 
Particle Fluid Surface node that creates the surface has the render flag (purple circle) so it is used when you render. You can click the display flag on the Particle Fluid Surface to view the surface in the viewport instead of the particles.
Tip
If the surface is too rough, you can try connecting a 
Peak surface node to the Particle Fluid Surface output to shrink the surface inward, and then add a 
Smooth node.
Tip
You can clip the fluid surface using any container and collision geometry, to prevent interpenetration. Import the container geometry into the fluid surfacing object, use an 
IsoOffset node to convert it to an SDF (set Output type to “SDF Volume” and use the Uniform sampling parameter to control the resolution). Then, connect the IsoOffset to the second input of the Particle Fluid Surface node. On the Particle Fluid Surface node, click the Collision Volumes tab and turn on Subtract Collision Volumes. You can use this to subtract any geometry from the surface, for example to add air bubbles.