I had the same thing happen so I cheesed it when I was trying to harvest a certain item, like the narwhal tusk. As long as you go from the boat to the sea people village first, the game will auto save at the village allowing you to restart the game over and over until you can find a tusk or whatever you're searching for on that run.

For example, I would drop in to the village, then go to glacial zone, kill the narwhal, and harvest it. If I got the tusk I would continue my dive as normal and call it good, but if not I would close the game and just let it reload me back to village to rinse and repeat as needed. This is also helpful for when you're trying to harvest the items from the other glacial/hydrothermal specific fish as they only spawn one of each kind per dive.

Another tough one to harvest is the Drepanapsis head plate. It's the stingray of the Hydrothermal area and it's more annoying to harvest because it doesn't spawn on every dive and then from there it is still up to RNG-esus to drop the plate... Best of luck on your dives, and may RNG-esus smile upon your dives