How are the current ocean depth and ph levels monitored? Could a device be built that would sink to the bottom of the ocean and record data then after maybe a year inflate a float to bring it back to the surface to broadcast the data? It seems like you could just plant these all over the world.
There are devices something like what you suggest, but they don’t stay down for a full year or stay on the bottom. Here’s one example, but there are others. For a general discussion, see Autonomous underwater vehicles
With regard to depth, I’m not exactly sure what you’re talking about, and what you envision needs to be “monitored”. There are various high tech methods for mapping the contours of the seafloor, and the history of how these techniques were developed and how data accumulated is integral to the story of the development of the theory of plate tectonics. And obviously seafloor tectonics are a dynamic process. But I hope you realize that sea level changes are not monitored by measuring from the bottom to the surface…
that was a misprint, I meant temperature
NOAA maintains an array of bottom mounted pressure sensors in the Pacific for monitoring depth-actually pressure to record long wavelength waves in the ocean.
As for the devices the OP asked about, there are thousands-see the ARGO array.
Argo (oceanography) - Wikipedia. They don’t stay down for a year, I don’t see the point of that. One place that changes VERY slowly is the deep ocean. Measure conditions once a decade and you will be over sampling.