Tidal Affinity is a Exploration skill in Torment: Tides of Numenera.
Unlocks more difficult Tidal Surge options.
Allows the Last Castoff greater control and finesse with the Tides, granting access to increasingly complex ways of manipulating people and objects with those forces with Tidal Surges.
Another chance to learn the skill comes from Melmoth Leviarm in Miel Avest, once Aadiriis directs you to speak with him about the Tides. This will not upgrade it to the Specialized level here if you already have it from Tol Maguur.
The skill's level can be raised by speaking to Kwherezimian in the Bloom.