HR sensor not working after 12m diving

After a session of diving(12m pool water) the HR sensor stopped working.

I opened the watch 4 and no water damage or indicator marked.

Where is the HR sensor located and what can I do? (I suppose in the backplate but the green light works, so maybe a part of the sensor?).

Also seems not to be parts for this available.

Factory reset done with no difference.

Thanks in advance