I think that you are
If I eat you, you might eat me and then you won't ever be hungry again.
If you are submerged in water and drinking it in you are probably drowning so will soon never be hungry again.
If I eat you and you don't eat me, you might have dressed up for me.
If you are submerged in water but not drinking it then it indicates you have planned to swim or scuba dive, so will be appropriately dressed.
If I eat half of you, I might not be very tall.
If only half submerged, you must be standing or sitting in shallow water.
If I don't eat you when you eat me, I'm small, but not as small as when I fall.
Water you drink comes in larger portions than when it falls (rain).
If neither of us eats the other, I will still be waiting for you in your bathroom.
Water waits in the pipes and toilet of the bathroom.
I'm a serial killer.
Many people have drowned and continue to do so.
But you like me.
We all like to drink water in some form.