[edited after feedback in comments] I've had an idea for a story that takes place on a planet that is primarily shallow oceans (average depth of no more than 100m or so, max depth of no more than 1000m). In general, the story I'm trying to tell would be set on a world just barely on the edge of habitability. I want it to be harsh or difficult in some way.
The planet would have no plate tectonics (like Mars). The only land masses would be volcanic islands formed similarly to mid-plate volcanic islands (Think Hawaii), with at least one island that is very large, like Olympus Mons (which is also a shield volcano like the Hawaiian islands).
The planet would be tidally locked around an M-class star. So not only would the inhabitants be spread across these small islands, but there would be a relatively narrow band of habitability and likely some pretty harsh weather, especially winds.
Otherwise, the planet would be similar to Earth in most other ways that count for habitability (mass and gravity, atmospheric composition, etc.)
My question is, could a tidally locked planet with no plate tectonics plausibly have some way of generating a magnetosphere or some other mechanism for maintaining an atmosphere? Would such a planet be habitable?
I have also read some research recently around tidally locked S-type planets in binary systems that suggests such a planet orbiting an M-class star may be more habitable if the M-class star system was orbiting a larger G-class star similar to our sun. The added irradiation would help raise temperatures on the dark side of the locked planet, and would create some kind of variation in illumination even if it would never truly be "night."