@mprogers, thank you for the additional information.
I have thought about your responses and have a few thoughts.
Another thread related to solar Boron projects that is waiting on the roll out of 1.5.0 final vs. the current 1.5.0-rc-1. Once this update is rolled out @Rftop has been leading the charge on providing excellent data regarding power use in sleep modes and power budgeting. I would watch this linked thread for new information.
I do not have any experience with the GPS module, so I cannot speak to power requirements of this, so maybe you can specify which one you are looking at. I can point you in the direction of a great tutorial written by @Moors7 that combines a Boron and a GPS featherwing. This article can definitely provide some inspiration, and this feather wing does have a documented power consumption.
Further thinking, knowing you are tight on space and power I started to kick around the idea of making the setup solar powered. You mentioned that the turtles will sun for hours, which I interpreted as a huge opportunity to leverage the turtles innate ability to seek and find awesome sun which in turn would lend itself to solar charging. I understand the desire to use the H2O sensor to detect absence of water (therefore at surface or out of water so attempting transmit makes sense); however, it was recently pointed out a Boron can wake from sleep when the panel begins to provide power. Also, inherently if the panel is getting sun there could be a likelihood the turtle is at surface or sunning (I don’t know if we are talking clear water or murky).
Lastly, the enclosure dimensions are going to be challenging. Knowing that any water intrusion could trash the whole setup enclosure selection is going to be key. I tried doing a bit of research finding something that was IP68 rated and plastic (so cellular signal can pass through) and everything was huge. I did find something like this enclosure but it is outside of your size constrains. Either way I don’t have a good answer for this but this could be a challenge.
The above is food for thought, what do you think?