Mine stays unlocked mostly the whole time that I'm wearing it and I only have to unlock it if I take it off. Generally this is in the morning when I take it off the charger and I also use the iPhone unlock feature to unlock it rather than typing in the code on screen.
I often wear mine loose, particularly when I'm playing the guitar, because otherwise I get an ache in the abductor muscles of my hand. Occasionally when wearing like this it will lose contact and I have to put in the code or unlock with my phone but this usually means I didn't drink enough water, I'm dehydrated and my wrist shrank down that little but more.
The lock on loss of contact with the skin is supposed to be THE security feature against the watch being stolen and Apple Pay being used on the watch.
FWIW: I usually have mine set on the 6th hole from the end on the Black Classic Buckle but occasionally I set it on the 7th hole if it's flopping about and I take that as cue to drink more