I did this once, in about 7 feet of water with Hampture Mk. III. As I had only a snorkel and mask, deploying it was miserable, and recovering it was downright dangerous for me. That was part of my motivation to get scuba certified. 7-8 feet is just barely deep enough that, if you're hauling a big heavy object out of water, you're potentially endangering yourself. But with a small scuba setup, deploying and recovering a habitat to/from 8 feet should be much less miserable. As you can see in the diagram below it would be supplied air and power from a floating plastic storage bin, containing one 28ah agm battery, a 175 watt inverter, the 110v->5v2a adapters that power the heated pads, and the deep water aquarium pump. This setup is sufficient to supply air and heat for around 10-15 hours. I expect that I'd deploy it for around five hours, and call it a day. This should be ample time to take photos and video. I've also identified places nearby to submerge it with much, much better visibility (clear water!) compared to the pond I put Mk III. in. It'll be an inland lake or canal, not the ocean, as I need relatively calm water for safety reasons. "Deep lake" doesn't have the same ring to it though.
8 feet is about the point where you feel the need to equalize. Depth doesn't "scale" for hamsters (i.e. just because they're smaller doesn't mean 8 feet to them is like a thousand feet to us or whatever) they experience the same rate of pressure increase by depth as humans. So this is about as deep as I am comfortable deploying them, as the pressure on their inner ear should be gentle, and easy to equalize even for an animal not accustomed to having to do so. For added comfort I will be deploying the habitat very slowly, at the rate of two or three feet per minute, only possible now that I have access to scuba gear. Likewise when I retrieve the habitat: A slow ascent, to allow their eardrums to equalize gently on the way up and to ensure that they don't overexpand their lungs. I expect it will be remarkable to see in nice, clear water, for the habitat which is normally plunked down in a 2 foot deep aquarium to instead be so deep that I have to fully submerge myself to take pictures/video.
As I already have all the parts needed for this, except the battery, it should be doable some time in the next year. And since doing anything more ambitious than this costs exponentially more than what I've done so far, I think once this 'deep sea' mission is complete, I'll call it quits. The Hampture project will then be considered successfully concluded. As the hamsternauts are getting pretty old, I'll be lucky to do this before they die of old age, and combined with the Skyhab project they'll have lived remarkable lives few humans can identify with let alone other small mammals. I'd fashion them some tiny little medals but they'd just eat 'em.