If an effect says that an opponent may cast a spell during your turn, Tidal Barracuda’s restriction overrules that permission.

Tidal Barracuda’s first ability doesn’t grant flash to any objects. It just changes when spells can be cast.

Tidal Barracuda’s abilities apply only while Tidal Barracuda is on the battlefield. Notably, its first ability doesn’t apply to Tidal Barracuda as you’re casting it.

Your opponents can still activate abilities during your turn.

Once a player announces that they’re casting a spell, players can’t try to remove Tidal Barracuda to make it so that spell couldn’t be cast at that time. Spells cast as though they had flash will still resolve if able, even if Tidal Barracuda is no longer on the battlefield at that time.