Yes. All of my yes. Take nothing but the yes. Shut up and take my yes. I'd really like more of this in the game. Specifically, the following:
Soccer. I don't even like the sport, but I feel like it'd be fun in the Sims.
Martial Arts. This was really fun to max out in the Sims 3 and it was fun watching my Sim train other Sims in the art. With the smoother animations of ther Sims 4, I really hope this makes a return.
Fencing\Swordplay. They could look at the Sims Medieval for ideas. In that game, you could even kill other Sims with swordplay.
Boxing. I don't know. As I mentioned, there was a game that allowed Sims to kill other Sims which was the Sims Medieval. And with Martial Arts, they do hit each other. I'd like to see this since it feels so odd to me you can only use boxing skills with the bag. Why add it in if it can't be used for what its originally supposed to be used for?
Breakdancing. Just because I like watching my Sims dance.
Archery. It would be cool, but I wonder if it's going to be any more interesting than just watching a Sim shoot at a target.
For the open water activites, if we ever get the ability to swim in open water, I agree completely.
Rollerblading.
As for more hobby ideas in particular, you could do the following:
Sketching: I really miss the sketch pad of the Sims 3 University and personally, I think Sketching is a very beautiful thing to learn.
Gambling: This could be introduced in some kind of Late Night EP.
Urban Exploration: This could introduce a more urban city world where your Sim can explore tunnels, bridges, etc.
Caving.
Scrapbooking.