Apple Watch Storage is Full

Unpair your Apple Watch via the Watch app on your iPhone.

Then Re-pair your Apple Watch again,this usually frees up

enough storage space to update your watch.And without any loss of data.

Or this;

If you don’t have enough space to update your Apple Watch – Apple Support

Curiously, I started getting this "Storage Full" message on the same date.

As with original question, I had no photos & no music on the watch (series 2, up-to-date OS), and definitely not 4.7gb of anything knowingly stored.

I followed the unpair/re-pair route a couple of times (which I've done a few times in last couple of years), but re-pairing didn't complete the cycle, stopped during the 'sync' stage. I then used the 'reset' option to set up the watch from scratch again....this worked, storage now showing 720mb used, 4.0gb available.

Follow these steps:

Unpair and erase your Apple Watch - Apple Support