go to the scene with the directional emitter and select you particles.., go to 'fields ' in your menu and select 'gravity'

that will help them to 'go down'

use blobbys.., mess with the thresh hold (press render attributes' in the AE particle shape node attributes ( the AE).., render and mess with the thresh hold until you get a watery look.., if thats too much.., stick with the default particles (points)

constraints for particles?? beats me.., just mess with the fields.., look at the options you have in the channel box for the emitter., spread, speed, evt