Here is a brief description of a weekend (September 14 to September 16, 2007) at the Lake Thun.
We left on Friday afternoon and arrived in Interlaken about 4 1/2 hours later. We had reserved at Hotel Aarburg. Originally, we had thought to book a hotel farther away from the lake. However, we did not want to risk large altitude changes right after diving. For that reason, Interlaken seemed like a good alternative, even if it is a bit more expensive. A single room at this hotel costs 75 CHR (=50 EURO). It is very clean, centrally located, the breakfast is fine, and late in the evening we still got a good, cold plate of cheese and sausages. They also told us about a drying room. Thus, in winter there exists the possibility to dry the diving equipment. The owner seemed to be somewhat stressed at times. It may be wise not come up with extra wishes like breakfast on the terrasse, and breakfast should indeed be finished by 10 am. But all in all the hotel was good.
On Saturday we dived twice in Lake Thun. We found good information about dive sites on http://www.kohmann.ch/tauchen. And we could download detailed sketches of dive sites from http://www.maertex.ch/tauchen_thunersee.htm. Early in the morning we dived at Fishbalme, in the afternoon at Ameisenegge. Supercalifragilisticexpialidocious!!! At this point in time (mid September) the water was a bit hazy down to a depth of 20 meters. But further down the visibility improved steadily, and latest at 30 meters the visibility was great. The steep-faced walls are overwhelming. At Ameiseneggen we very much recommend the "Grand Canyon". For sure we only saw a small fraction of these diving sites. For that reason, we'll be happy to dive again at these sites sometime in the future. Parking at both places was no problem at that time of the year. At the Fischbalme we were the first at 7:30 in the morning.
The diving flag is important when diving in Switzerland. And indeed every group of divers we saw set up their flags. In Switzerland the flag Alpha (blue/white) is required. We carried the international flag (red/white), but were not fined.
One can fill diving tanks at the so-called Tauchertreff (http://www.tt-thunersee.ch). However, they only open on Saturdays up to 4 pm and are closed on Sundays. But if one carries a filling-card, then one can fill diving tanks at their outside filling station.
Unfortunately, we did not have a filling-card and finished our second dive too late for filling. For that reason, we used Sunday for a lengthy hike, which was really nice. We were really lucky with the weather. In summary: Diving to Lake Thun only for diving really is a bit of waste. We drove to Habkern, which lies above Interlaken. From there we could hike for many hours. And for lunch and relaxing we can highly recommend Alp Heubuehlen, where one can enjoy home-made cheese.