It is difficult to test the hat directly without another RS485 device to communicate with. You can test the UART port on your Raspberry Pi by connecting the GPIO14 (TXD) and GPIO15 (RXD) pins together on the Raspberry Pi GPIO header without the RS485 Pi connected and use a program like minicom to send data out on the ttyAMA0 port. With the TX and RX connected together you will get a loop back into the Raspberry Pi so anything typed into minicom should be echoed on the screen as it is received on the RX pin.
Another way to test the UART port if you have two Raspberry Pis would be to connect the TX on one Pi to the RX on another and try sending data between the two Pis. You will also need to connect the ground pins together for this to work.
Once you know the UART port is sending and receiving data you can narrow down the problem to either the RS285 Pi or the water level sensor.
Do you have a link to the water level sensor so I can find the datasheet for it and check that it is compatible with the RS485 Pi?