I've searched high and low, and I can find not Facebook Messenger or WhatsApp apps for Wear OS on the Galaxy Watch 5 (or the 4, which I previously owned). I'm a prodigious user of WhatsApp, and I use my watch alone over LTE every day when I go out for a run, so I would love to have this feature. Alas, it does not exist at the moment.
Best you can do is respond to WhatsApp messages via the notification that shows up on your watch. So you unfortunately can't initiate a WhatsApp message, but you can respond to one. Not ideal, I know.