Easy in one way, hard in another.
Why?
Every year kids Drown in pools. Sometimes because they compete in swimming underwater for longest distance, or to be underwater the longest time. Most kids are smart and some smart ones figure out that its easier to win over your classmates If you hyperventilate first.
What happens next is that some kids Drown by doing that. The reason is that they,by hyperventilate, expend so much co2 from their blood that it takes a long time for the co2 level to rise in your blood and trigger your breathing reflex (reflex is not the right Word here. Exuse me. i am swedish.)
The amount of oxygen in your lungs/blood is not what triggers your feel of lack of breath. Its a high level of co2 that does that.
So for instance, hyperventilating and then just before jumping in the water you fill your lungs with, lets say pure helium from a ballon. Then drowning yourself with no Weights is possible.
If you have not trained yourself in freediving( withstanding high co2 level and the mental pain that Comes with it)drowning yourself in a bathtub Will be very hard to do.
But there are more options. Throwing yourself into the ocean a stormy night from a boat far out Will do the trick as well. It Will just certainly be very painfull.
And one more note on the subject of pain.. The pain felt while drowning oneself is horrible... But its nothing, and I repeat that, nothing compared to the pain being felt by the ones left behind, Mothers, fathers, sisters, brothers, children, loved ones and friends Will feel a pain much greather than that of the person drowning him/herself.