I think the deeper (pun intended) meaning or lasting message of The Shallows is obviously a metaphor for her mother's cancer and her going through the same, or similar experience.
- Cancer, when it comes, is like a shark circling round you - it's always there, and never goes away unless you do something about it. It won't go away on its own, and you're more likely to die of dehydration/cancer if you do nothing, than risk the shark
- At first, you're not even aware that the shark/cancer is there
- When you're in medication, especially chemotherapy and radiotherapy, you're very limited to where you can go and what you can do - like an island. Sometimes you can feel alone too.
- Cancer can leave scars - not just physical (the one she left with) but also mentally
- The medication can take the life out of you - you saw the colour leaving her lips and her constant exhaustion
- A lot of people going in for treatment have someone or something they keep with them - in this case it was Steven Seagull. It is good to have some comedy to keep you tethered to reality.
- You lose your hair through chemotherapy - although in this case she lost a massive amount of blood
- Some people die from cancer/sharks very quickly, i.e. the other divers that were instantly killed
- The dead whale is a suggestion that cancer can affect anyone regardless of their background, genetics, diet or health