If I had a Fizz in board and a Tide Turner hidden, and my opponent attacks with Fiora Peerless, and I react to the effect with Tide Turner, does Fiora have double might still?

Yes, Fiora Peerless will still have her Might doubled.

According to the rules and related FAQs (specifically FAQ #1231 and #577), Fiora’s "When I attack" ability is a triggered ability that checks its condition—being "one on one"—only at the moment the ability is placed on the chain.

Here is the timing sequence:

1. Initial Chain: Fiora is designated as an attacker. The battlefield is one-on-one, so Fiora's trigger ("double my Might") is placed on the Initial Chain.
2. Closed State: The game is now in a Closed State. You play your hidden Tideturner as a [Reaction] to Fiora's triggered ability.
3. Tideturner Resolution: Tideturner resolves, swapping its location with your other unit. This changes the battlefield state.
4. Fiora Trigger Resolution: After Tideturner finishes resolving, Fiora's trigger finally resolves. Because the condition ("one on one") was already met and validated when the trigger was initially added to the chain, Fiora's Might doubles regardless of the fact that your units have since swapped locations.

As noted in the Showdown Flow and Rule 359.3.f.4, "When" triggers are evaluated at the time of designation and addition to the chain, not at the time of resolution. Revealing hidden cards or moving units after the trigger has already been placed on the chain does not retroactively prevent it from resolving.