Has anyone managed to get temperature to one decimal on the app? It shows as one decimal place on watch but not the app. I use it for cold water swimming and would be really helpful. Thanks

Yeah, you can use the free site https://www.runalyze.com, which syncs with your Garmin Connect account.

If you roll over an activity's temperature graph with your mouse/trackpad in runalyze, it shows you the value to one decimal place.

Only downside is that the site works best on a computer. If you use a phone to access the site, the user experience won't be great and some functionality will be unavailable. (The graph roll over functionality pretty much requires a mouse or trackpad - it's not like some sites which also work properly with touch, for this kind of thing.)