A stone will not need you to guess if,
You’re still going to drown.
What is the meaning of the above lines in the context of the song?
The lines mean:
If you have a stone, then you no longer need to wonder about whether you're still going to drown.
That is to say, if there is doubt about whether you're going to drown, then a stone can take away the guesswork. The reference is to the habit suicides have of weighing their pockets down with stones so that they cannot float to the surface but drown. Virginia Woolf died in this way.
The song is about how we're manipulated into believing superficialities ("illusion & dream"). The speaker is trying to convince someone else to notice this manipulation and to opt out of the "world of make-believe." We are already "drifting aimlessly." The images we are served up, which "dress" dishonesty "in a pretty gown," are the stones that will ensure that we drown in the sea we're already adrift in.
The speaker says we need to stop drifting and confront our demon. Either we chase the demon away, or it will take our freedom. Rather than participate in the noisy and restless world of images, we should strive to break away toward silence, peace, and freedom:
With silence comes peace
With peace comes freedom
With freedom comes silence.