Adafruit MPRLS Ported Pressure Sensor Breakout - 0 to 25 PSI (Bought from PiHut in the UK)
Seems to be working quite well. (Telemetry output is now signal strength, depth and battery voltage.
LATER EDIT: (January 2022) Still working well after a number of sailings. Surface telemetry value = 3. Dived to 0.5 m = 8