Use forces like you thought. Vortex field? Hum, I really would use the Volume Axis field (VAF). Now it has both a magnitude and a value "Away from Center" - crank this number - try 10, then 100, then 1000! Position this VAF in the middle of your water bomb.
If you keyframe the magnitude from 0 to say 10 when you stop your collisions, with away from center value - BOOM! Things should fly away!