When checking the Mosquitto broker log in HA, there is a connection from the IP address of the watermeter.
2024-08-11 02:43:37: New connection from 192.168.x.x:56394 on port 1883.
2024-08-11 02:43:37: New client connected from 192.168.x.x:56394 as D0_EF_76_5C_BF_A8 (p2, c1, k15, u'mqtt').
MQTT devices do not automatically get added into HA when information is published to any random topic on your broker. MQTT discovery is a process where the device must publish specific configuration information to a special discovery topic which describes how HA should add the device. If the device itself doesn’t publish this HA-specific information, nothing will happen.
It looks like your device probably doesn’t know anything about HA’s MQTT discovery, and therefore you will have to manually add the MQTT device