If she’s got an iPhone 6, there’s no point really to owning an SE. Both units are relatively easy to sell on eBay/Kijiji, so I’d probably go for the watch. The Series 1 is still a current model — Apple discontinued the Series 2 but still sells the 1.
If she’s at all into fitness, it’s a no-brainer. It’s better than most of the lower end fitness trackers for everything except sleep tracking (there are some decent apps, notably AutoSleep and Pillow, but Fitbit’s HR line is still better there, as is the now discontinued Jawbone UP3). You can swim with the series 1, especially recent revisions, despite what they say about it not being waterproof. The exercise app doesn’t support swim workouts on the series 1, but there are third-party apps that do. A lot of the competition like the Alta HR isn’t safe to use while swimming.
Important tip though: it will come with WatchOS 3. If she thinks she’ll use it to listen to music a lot on her phone, don’t upgrade to WatchOS 4. WatchOS 4 removes the ability to browse your phone music library. Now for phone music it’s basically a souped up remote control, and a lot of the stuff you used to do through the interface is now only available via Siri. If she’s a Spotify subscriber though it’s no big deal; not much changes and it’s arguably easier to use. Eventually there’s a good Spotify client app coming; the Spotify company hired the main developer. No word on ETA for that.