The Mark VI.

Another completely infeasible fantasy, due to the cost. I had the idea while looking for companies that sell transparent plastic domes in various sizes. The largest I found was 8 feet in diameter and something like $36,000. Presumably that's due to the low manufacturing volume, as people don't generally have a widespread, burning need for 8 foot diameter transparent plastic domes. However, were my funding somehow unlimited, this would be my ultimate masterpiece/cry for help:

Floating halfway between the surface and the ocean floor, anchored by steel cables in water 500 feet deep, the Mark VI would be positioned on the periphery of the gulf stream so as to sap energy from the currents via it's pivoting turbine, but not be subject to significant stress on the cables. The panels around the bottom of the shaft are passive silicone membrane CO2 exchangers; They refresh the air inside without using any electricity. In fact, the first experiment demonstrating this phenomenon was done using...an underwater hamster habitat! :O

As you can see, the one wall of the enclosure made up of the super thin silicone membrane isn't terribly large compared to the animal it's providing oxygen for. I simply decided to overdo it a little so that this hovering "seastation" could accommodate larger numbers, and go for longer periods between maintenance. Of course the plants help too, but it doesn't contain nearly enough to do the job without external support. The enclosure itself is 1 atmosphere (due to the depth, which insulates it against storms) contains a closed loop biosphere to provide the hamsters with food (plants, insects, seeds) decompose their waste, and by way of one of those tacky little multi-tier faux rock indoor fountains, a source of fresh water (passively desalinated by equipment in the lower half of the enclosure) that does not sit stagnant and allow insects to breed therein. Suspended from the top of the fountain is the artificial sun; A heater bulb, as used in incubators, provides heat and light that simulates natural sunlight exposure. Around the base of the fountain you may notice openings for various webcams that transmit live footage of the habitat interior over the internet. Finally, there's a docking hub just above the turbine section that permits an autonomous model sub, purpose built for this station, to dock to it and deposit additional food and chews should they be necessary, as well as to add additional hamsters (the sub carries a small compressed air supply sufficient for the round trip and is refilled after each use) and possibly to transfer 'crew' between multiple stations. If I ever win the lottery, expect an ocean full of these. Alternatively, if anyone from Pixar or Dreamworks is reading this by chance, it seems like all of this would fit neatly into one of their movies. Animals doing things they don't normally do when watched by humans, their hidden world cobbled together from junk, it's Secret of NiMH underwater. Call me, let's make this happen.