So,
@Liselle, where do I see opportunities?
Let’s start with big picture stuff. Story structure.
1. This feels like part of another story, not a complete story in itself. We don’t know who Astora hitched a ride with, how she encountered pirates, why they put her in the pit, whether they knew about the crab, what happens to the crab, or how Astora gets off the island.
1b. Even if we stick with the exit from the cave as the very last moment, and if this is a stand alone story, then I think the final line needs work: “...she climbed out the mouth of the tunnel and looked around.” One solution is tie this to an earlier passage. thinking about the lovely “stars” passage, a simple change to: “...she climbed out the mouth of the tunnel into freedom, and a sky of stars she swore she would reach again.” (If this were a SF story

)
2. I think we need more conflict. One way to do it, would be to bring the pirates “on stage”. Let them jeer, laugh, trade insults, threaten, and anticipate the fun of Astora’s death - panicked drowning or gruesome butchery. Another trick is to use try-fail. Astora could try something, fail. Try again, fail worse; in fact, make her situation worse. Finally, try again, and succeed.
3. Astora, as our main character, is the one whose actions should matter. I think you’re almost there with her trying to get the crab to snip her bonds. But, really, the crab, for no obvious reason, is the initiator of the action, and Astora is the responder. I’d encourage you to think about how to do this, and still retain the twist that the crab will help, not hurt: perhaps Astora has an insight that the crab is more nervous than she, maybe it eats metal and Astora is inspired to suggest the chains snipping.
4. Getting a little more detailed, you’ve done a good job putting a deadline on the action - the oncoming tide. Since I’m suggesting putting some More framing onto the story (why she’s traveling; why she was imprisoned; how she’s hoping to continue her travels if she gets out), there could be more intensity to the deadline. Just by way of example, not suggestion, a rising tide could open a way for the ship she was on to escape the rock-encircled harbor of the island. Not only does a\the tide threaten her with drowning, it also takes away her one way forward. She’ll be utterly stuck, marooned. How about them apples?
Finally,
5. Having the crab disappear into its hole and from the climax of the story feels... off. It’s such an important player. Short of the too-obvious “crab leads Astora to the exit”, what else could happen that involves the crab and leave Astora as the key mover in the tale?
Also, finally:
6. The exit from the hole feels long compared to the rest of the tale. Is this the climax, or is the crab freeing her the climax? Obviously, we need both events.
See the next post (maybe a little later) for annoying word-smithing suggestions, as though I’m a pro editor or whatnot.