Thanks for your query,based on the facts that you have posted it appears that you have blister
filled with clear fluid at tip of your penis.
You have not mentioned whether this followed after unprotected sex or otherwise.
Generally blisters at the tip of penis are due to Herpes
2 genital infection
that get transmitted through intra vaginal sex with infected female partner..
Please consult Dermatologist
and get swab test and culture of a fluid from blister done to confirm the diagnosis.
You may need to take anti viral medications like Acyclovir
for 2 weeks .
Ensure to wash your genitals with warm water twice daily
Ensure to avoid sexual encounters till it heals up completely.