Could also be master cylinder or just air in system.
When you say "Clutch pedal is in the floor" what exactly do you mean?
Is the clutch peddle always near the floor, or is it near the floor when it bites?
Is there a lot of free movement in the peddle?
If you are re-sealing the slave cylinder, might as well do the master at the same time, and change the fluid and bleed thoroughly. Only a few pounds extra, about 1 hour additional work.
As Dert says make sure clutch arm is free and not seizing in the housing (common problem).