I've heard relatively good things about diving around Zurich and have seen divers in Lake Geneva. I swam in Lake Luzern recently and the viz was OK. I suspect that, if you like tropical diving or cold water wreck diving, Swiss diving is going to be limited. However, these seem to be the positives:
- Diving in the Lakes gives you the opportunity to keep your skills honed ready for your next dive holiday.
- You get a chance to check out your kit ahead of a dive trip.
- In summer and probably Autumn and even late Spring the water is warm enough for diving in a wetsuit, nicely complementing the ski season.
- If you live here, it's the only possibility for diving at the weekends given you can't fly and dive.
- It's just a lot of fun being underwater even if there isn't a lot to see, especially if you can find somewhere with good viz.
- It can be quite social.
There is a dive shop in Basel, but it is in the process of closing down. The owners are retiring. It is located next to the Tram 1 and Bus Route 36/50 stop at Kannenfeldplatz. They have a lot of stuff heavily discounted last time I went in.