Vivoactive 4s stopped working, now stuck on charge symbol

Hi, my Vivoactive 4s stopped working a few days ago after I went swimming in the sea (something I'd been doing for a week with no issues). I gave it a good rinse out and have plugged in the charging cable. I get the blue triangle and then the white battery charging symbol when connected to the charger but it still won't charge.

Have tried all the reset ideas and changed cables etc but no joy.

FWIW I went on a two month holiday to Australia where I swam in the sea almost every other day and the watch stood up to it fine.

Any suggestions welcome. Only had it for three years, so seems a shame to junk it. I bought it off ebay so not sure Garmin would be able to step in.