If "undercurrent" refers to a hidden feeling or trend:
* Overt: Something that is open and obvious.
* Explicit: Clearly stated or expressed.
* Surface level: Superficial or not going deep into the subject.
* Mainstream: The prevailing trend or opinion.
If "undercurrent" refers to a current of water below the surface:
* Surface current: The current of water at the surface.
* Above-surface: Located above the surface of the water.
If "undercurrent" refers to a feeling of tension or unease:
* Harmony: A state of peace and agreement.
* Comfort: A feeling of ease and contentment.
* Relaxation: A state of being calm and at ease.
The best antonym will depend on the specific context in which you are using the word "undercurrent".