my watch slips because of sweat

Is there a reason you're wearing it above the bone? I wear mine in the natural hollow just below the bone, towards the elbow, snug and never have an issue with HR?

Wear a chest HRM? Honestly, if you're going to rely on the OHR from the watch then I've had success with the technique that AJRykala identified for stead state running.

Wear the watch tight.

Hello,
there is not enough space between the wrist bone and the wrist to wear the watch comfortably.
HRM chest do not stop my watch moving.
It's not only a probleme with the heart rate, when my watch slips, my wrist being finer towards the hand, the watch moves. I have to go back up all the time

Wear the watch snug tight, below the bone (towards the elbow end not the hand end) because the arm naturally dips there it's the normal place to wear a watch. Wearing it between the bone and the hand is not going to work.

ELBOW ===================WATCH===WRISTBONE==HAND

AJRykala, it's what i do.
but when i sweat my watch slips here :
ELBOW ====================WRISTBONE==WATCH=HAND
I have a tiny wristbone also...

I just wear a small wrist sweatband to stop it sliding down.

ELBOW ====================WRISTBONE==WATCH=HAND

That's what I do too. Keeping the watch above the wristbone feels weird in my arm. I just tighten it a little when running and it works fine.

I had a similar problem, though mine was to do with the watch slipping forward and banging on my wristbone. In an attempt to reduce this problem, I bought one of the silicone 'bumper' covers for the fenix 5 from ebay.

Google "Silicone-Rubber-Protector-Case-Band-Cover-for-Garmin-Fenix-5X-Fenix-5-GPS-Watch" and "ebay" to find it.

It helped reduce the problem two ways. 1) It softened the edge of the watch so that banging on the bone didn't hurt the skin so much and 2) The silicone underneath the watch seemed to reduce slippage, even when sweaty.
The case itself is quite thin and fits well. Doesn't make the watch much bulkier or anything and the buttons work fine through the thin silicone. There's a hole for the heartbeat sensor underneath but apparently enough silicone underneath to cover the slippery metal and lift the heart sensor away from the skin so it isn't acting like a slippery spot. (Plus there's a bonus cover for the recharge pins). Hopefully the HB sensor still works properly...

I'd recommended it for protecting the watch, but even more for protecting the wearer.
Plus, now I can do things with the silicone under the watch to reduce slipping even more, such as roughen it with sandpaper, or put silicone ridges or dots on it. I didn't want to do that to the watch itself.