When can you activate Tideturner from hidden, given that hidden cards can normally only target cards in the same battlefield?

Ruling: Tideturner can be activated from hidden to target units on the other battlefield when all units on its own battlefield are illegal targets, functioning as a reaction.

Sequence:

• Check if all units on Tideturner's battlefield are illegal targets
• If so, Tideturner becomes a reaction
• You can then target any unit on the other battlefield

Nuances:

• This allows you to save a unit on the other battlefield during combat
• Can also be activated in response to a spell (though less commonly relevant)