The long initialization time could be the result of large amounts of geometry. If there is a large number of whitewater particles (more than ~6million for example), this could be a contributing factor to the long initialization time.
By default, the addon will instance icospheres over each of the whitewater particles. This generates a lot of geometry and may greatly slow down the render.
There have been improvements in Blender for particle rendering. In Blender 3.1 or later, the whitewater meshes can be converted to Point Clouds which are much more efficient for rendering. This can be done by adding a “Mesh to Points” geometry node set up to the whitewater meshes:
More information on point clouds setups can be found here: https://github.com/rlguy/Blender-FLIP-Fluids/wiki/Domain-Attributes-and-Data-Settings#whitewater-point-clouds
Let me know if whitewater particles do not seem to be the cause of this rendering issue.