Yes, Cascade is the editor for particle systems.
Your water effect Blueprint(BP) has a ParticleSystemComponent(PSC) called Effect which is using the ParticleSystem(PS) P_Water_Projectile. Go to this asset in the content browser and copy it to a location in your game directory structure. Set you BP to use this new PS and open the PS for editing.
This will open up cascade.
At this point you should probably give the cascade docs a once over but simply, a Particle System is made up of one or more emitters. These are the columns in the main window. Each emitter is made up of various modules that control it’s behaviour.
To get the water effect to collide with the world, the emitters in this PS will have the Collision module applied to them.
Click on this collision module and find the properties ApplyPhysics and ParticleMass.
Hope that helps. If you’re still having trouble I’ll post some screen shots explaining this in more detail.
Cheers,
Si