Hi Matt,
Good to see you here in the Cayenne Community. I think the altitude is just used as an input to determine the pressure, but it’s not an actual sensor on the BMP180.
The fly fishing project sounds cool… perhaps we start with goldfish though 
@Ian seems to be the current community expert on aquatic Pi projects at the moment. TSL2561, BMP180, DS18B20 on a Zero - #5 by Ian
We will allow for users to change units, but there it is not possible right now unfortunately.
-B