Very Large Habitat Concept

This one's a bit "outside the box". It's a large ham ham colony based on this kind of inflatable underwater shelter:

As you can see, a few people have done this for their own use, and luckily for me when done the way they did it, it isn't especially expensive. The second one is of particular interest as it needs no weights! It is secured to a crevice in the rocky surface of the bottom. Wherever it's possible to do this, this method is ideal as 99% of the hassle of doing this sort of thing is weighing down all that air. Anyway, on to the main attraction:

This assumes no such crevice will happen to be handy for securing the thing and that weights (many buckets of sand I will fill underwater, then secure the sheet of plastic to, then release air from a scuba tank into it) will be necessary. As you can see, the hambro living space consists of the same type of cage they live in on land, made from stacked, bolted together transparent plastic storage bins with a climbing tube to get between floors.  

These simply float on the "surface" of the water inside the big bubble of trapped air. Holes in the lid allow fresh air to circulate into the cages, as usual, and fresh air is sent down to the inflatable dome shelter using the same oil free electric air compressor I used to supply my diving helmet. This will ensure an ample airflow suitable for more hamsters than I could reasonably fit in the thing.
  

Crucially, this lets me surface inside of it to open the cages, clean out the 'dirty spots', resupply food and water, etc. without bringing any of it up to the surface. And because of the more powerful compressor, this thing can be up to 21 feet deep if I ever want to surface the hams for any reason or with a yet more powerful compressor, up to 50 feet deep (the limit of using normal air before the oxygen content in it becomes toxic) if I was content for them to live down there for their entire lifespan (as I would be unable to safely surface them due to the nitrogen in their tissues).

I think I could fit about four floating cages in there and still have enough room to surface and maintain them. With 3 hams per cage, that's a population of 12. I could go higher but I can usually only find siblings in groups of 3 or less.

This would be very fault tolerant as well. The enclosures themselves don't have to be sealable or remotely watertight around the rims, just able to float stably. Overpressure inflatable shelters are in general pretty foolproof, just not commonly large enough for a person to live in. For hamsters however, it's massive, and if you were going to conduct an experiment to determine what effects living underwater for several generations has on complex mammals, this would be the way to do it.