Although I am rather "junior" in my Lego experience, I have learnt about a few "under water" sets that Lego had released in the past, while I still haven't opened the Aqua Raiders sets I own, I was lucky to get a wonderful 1997 set under the Town-Divers theme: 6441 Deep Sea Refuge.
Despite being an incomplete second hand set, since the price is rather reasonable, I see it as a very good deal, especially after finishing the build and seeing the finished product, it was quite a hassle to find replacement parts (which are more than what the seller mentioned but I think it was a honest mistake) and I had to settled with using parts of different (but seemingly compatible) colors, but I think I was able to come up with something that can reflect the value of the original set.
This decade old set contains a lot of unique types of parts not seen in recent years any more, such as the winch, portholed and various shapes of panels, there are many printed parts too. The attention to the details is also amazing - x-ray machine, storage area for equipments, knives.
The lab structure is built on two 16 X 32 Tan baseplates (which are very useful) hinged together, and can be fold and unfold to work on the interior, there are so much playability there.
The submarine itself is also already an excellent piece of vehicle by itself, the driver is seated inside a hinged dome, and the cabin doors can also be opened, equipped with the arms, it is very realistic as an undersea explorer that can collect specimens.
You also have a boat with winch, which can be hooked to the base. Please note that the back of the boat was originally a blue plate, but it was missing from the set when I got it, so I replaced it with a white one.
I think a lot of people looking at this set would focus also on the sharks and Stingray, in this second hand set one of the stingray and a shark is missing, it is still ok as I got enough creatures to keep the ocean floor busy. By the way, how easy is it today to get a Lego Stingray?
Please note the Underwater Scooter that the divers can use to move around, this is also a cool part of the set!
The porthole are very cool, making the interior lighting extremely realistic for an undersea feel.
The minifigs shown here may not be equipped as the same from the original set (I think even the cast is not correct either), according to Peeron there should be 4 clear diver masks in the set, but I got only 2 here, but overall this crew with the set as I got it is acceptable to me.
Comparing the Atlantis sets to this Deep Sea Refuge set, I think that Lego has not been exerting best effort in the new theme, having great minifigs/creatures like the Squid Warrior is not enough, the attention to details and being generous in enriching the content would be necessary to come up with a great set, if the quality of 6441 can be found in the Atlantis set, it could easily be one of the best Lego theme in 2010.