So the USB-to-serial adapter arrived. I can now program the ESP-01 module, considerably smaller than the development boards I have: the WeMos D1 mini, for example, is a little too big to fit inside the small space available in the plastic enclosure.

With a bunch of jumper wires I kinda managed to connect it to an ESP-01 module. Did a first programming, then I unplugged the wires from the adapter, re-plugged the ones for the fish, and checked if it worked. It did work, but I needed a few adjustments. Unplug again, re-plug again. And every time, carefully check for mistakes.

Mildly fed up by this tedious process, I decided to build myself an adapter board. So I could plug the ESP-01 on one side, and the USB-to-serial adapter on the other. An evening and a morning later, here's the result:

It's now WAY easier to program the ESP-01!
The USB adapter does provide 3.3V but I think it's not enough for the ESP8266 (I saw the voltage dropping under 3.1V). To stay on the safe side, I made the power come from my lab power supply.

The next step is to implement OTA programming, so I can keep programming it even if the ESP-01 is well tucked in the fish. This is particularly important because the space is so tight, I'll have to remove the 8-pin male header and any physical communication with the host will be a lot more annoying.