In the first scenario of innsmouth it says that I must gather all the cards from the following encounter sets. It has flooded caverns among them. I can find any any cards with this icon other than locations

The flooded caverns are all locations with the flooded caverns icon; there should be six of them.

Edit: All named Tidal Tunnel on one side - on the other side: - 2xUnderwater Cavern (card #102), 2x Tidal Pool (card #103) and 2xUnderground River (card #104).

The Flooded Caverns set consists entirely of locations.

Each card has the number of the cards in that set at the bottom of the card so all you had to do was look at that to know that you had all the cards.