Can Tideturner choose a unit that is already in base and swap location with it?

Yes. A base counts as a Location (Rule 146.1), so a friendly unit sitting at your Base is a legal choice for Tideturner's "another location" targeting — provided Tideturner itself is at a different location (i.e., a battlefield), which is how it enters when played from Hidden.

The swap is legal movement-wise: Tideturner can move from a battlefield to your Base, and the base unit can move up to Tideturner's original battlefield (subject to the usual destination restrictions in Rule 447, e.g. not into combats/battlefields involving two other players).

One catch: if Tideturner is played to your Base (not from Hidden), a unit already at Base is at the same location and cannot be chosen — the target must be at "another location" (FAQ 7937).