

Do you apply a considerable amount of pressure to your penis in any way? A water bump description would suggest this is due to an injury of some sort.
The area where this is is termed the Meatus, this is the outlet from your urethra: it is possible that it has developed this way, or is due to some form of injury, but it is also possible for damage to have occurred and there is an infection to the Meatus, or potentially to the urethra itself or as well as the meatus
The approach you can take it to leave it for 8 to 10 days and see what happens, it may heal itself.
If you masturbate try to abstain until this has completely healed, then add at least another 2 to 3 weeks of abstaining from masturbating after healing has occurred.
If it gets worse you are going to need to seek medical attention