swimming in fresh water is fine. you’ll want to use the ‘water lock’ feature to disable the touch screen, some people have the misconception that that feature somehow makes the watch water-proof or more water-resistant than when it’s off, but all it does is disable the touchscreen, and then when you turn it off, it’ll try to eject any water that got inside the watch.

keep in mind that you may not get heart-rate readings during swimming workouts, though, and that apple’s warranty won’t cover any damage resulting from water even if they do make claims of the product’s water resistance

it was absolute trash with 10.0, a lot better with 10.1.1 though not quite at what it was with 9.x

i don’t know, but Spotify will let you (can also control it from the Watch on tons of other devices)

you’ll have to wait for the start of the next month. it’ll show up in the Fitness app on your iPhone.

they’re ‘personalized’ based on your past activity, but generally pretty useless. used to be that they’d give you absolutely insane challenges that weren’t reasonable if you cared at all about rest, recovery and injury pervention, but since watchOS 9.0 or so, they’ve seem to have swung the other way and have always been incredibly easy to accomplish for me.

yeah, you’ll get the occasional erroneous reading

with the stock app, yes. with a third party app like Workoutdoors, you can have a much larger range of metrics to choose from and display.

i would keep in mind, though, that optical heart-rate sensors do struggle with getting readings while swimming in water and Apple notes that you may not get HR readings for swimming workouts.