Thank you, so many great ideas here, I'm still digesting all of them.

Great idea about using Lora. I just did some quick learning about it and this board looks good to me, but I admittedly don't know much about Lora (or deep sleep) yet. This one claims 50ua in deep sleep, does it look good to you?

https://www.amazon.com/MakerFocus-LoRaWAN-Development-ASR6502-Intelligent/dp/B089YFZQQK

(And mostly a note to myself: that 50uA in deep sleep applies only to v1.1 of the hardware, which is printed on the board, but some reviewers say they received v1.0 of the hardware, which consumes much more power, so make sure to get v1.1).

There's also this board, but it says 800uA in deep sleep.

Any other to consider? It's not a rush so buying from Aliexpress or similar is fine.

Or are you suggesting not to use an ESP32 at all? If so, got a specific devboard recommendation?

Depending on your housing material, and location of the sensor, you can likely just assume the housing has the same body temperature as the water.

Maybe. The water temperature varies through the water column, so I'd like to get it a few feet underwater. And the sun will probably heat up the enclosure. A difference of even 1 degree is significant for my purposes. But of course it might turn out after testing that I'm over-complicating things as usual and that the temperature inside the brain is the same as the water temp.

If you use connectors, find a sillicone dielectric grease and pack the connectors full to prevent water/contaminant ingress.

Good idea. If I leave a USB cable inside the housing, can I pack that with grease and then still use it to flash upgrades? Probably connecting to my laptop via a USB extension chord to avoid getting grease in my laptop's USB jacks. Or better yet I should probably just learn how to do OTA updates so I don't need to plug anything in...

I use this.

The only problem with it is that it doesn't have an ADC - so it can only do digital reads or PWM.