Close enough to geothermal vents and/or plate boundaries so that geothermal power is an option to help reduce energy costs.
I found this image showing the hottest geothermal regions:
Although I'm not totally sure if this covers all the random geothermal blobs far out at sea, it's probably easier to access the ocean floor if you're somewhere near land where the ocean isn't extremely deep. Most of the red blobs have islands or aren't even in the ocean, but the one between Portugal and the Island of Newfoundland doesn't. That might be a difficult place to build; according to this image from the National Geographic society, that's right over the Atlantic Mid-Ocean Canyon, with a rather inconvenient depth averaging several kilometers (2-3 miles-ish).
Close enough to the surface to take in some amount of sunlight for greenhouses, not be crushed under the pressure of the water around it (strong building materials should help combat this), and generally help people's circadian clocks stay somewhat stable.
However, it also needs to be far enough under the ocean to be hidden from the general public (governments really don't matter as much).
To fit both of these perfectly is a bit like trying to find a number both less than three and more than four. With satellite imaging and Google Maps and such, if there's little enough water between you and the air for farming off the sun, there's little enough space you'll get seen.
One possible solution would involve camouflage. Ideally, go somewhere with pretty crazy seafloor, and build your thing with a similar pattern. With good enough indoor lighting, you could probably farm inside. I'd say keep the humans on a mostly-plant diet; animals eat more than they produce and aren't really worthwhile in a compact environment, except for experimenting and occasional fancy dinners. Potatoes would make good underground-crops because they keep just about forever and don't require refrigeration. Also, farm stuff without dirt; use nutrient-imbued water, whatever that stuff is called.
A quick google search and this page
revealed that a sphere would be the best shape if it remains stationary. One, it's physically better, two, it keep the commutes within the thingamajig shorter. Of course, if it has to move, a sphere wouldn't be... hydrdynamic? I guess that's the word for that. Also, it'd likely just spin around, and you wouldn't get anywhere. As for keeping circadian clocks in line, there's not much you can do other than keeping the schedule clear at night and maybe making the lighting a bit darker (note use of darker) at night.
On the opposite side of the spectrum, you could make your lab totally public. (Or at least, it seems totally public.) An oil rig with a secret lab might work, although questions could come up. ("Say, Bob, how does three tons of cheetah food help you drill for oil?") One advantage of this is that it provides an energy source. Assuming, of course, that fossil fuels are still used when your story takes place (I dunno whether this is supposed to be futuristic or not). I couldn't get a map of where in the sea oil is common.
A variant of that, even more public, would be to do something with tourism. If you made an ocean retreat with an amazing zoo (Mr. Marglewithy's Magnificent Marine Menagerie?), then you could ship animal-keeping supplies (or the animals themselves) without making anyone suspicious. Chemicals might be a little tricky, but you could probably think of some excuse. This would, in my mind, be the best approach.
Possibly somewhere that ocean currents could be used to help generate power (I've heard of this idea, but it's kind of icing on the cake).
The feasibility of this is somewhat dependent on how well you can access the ocean floor. If you rig an ocean-current device off your lab, it's probably gonna drag your lab and all its energy will probably be spent keeping your lab on one place however, if you're able to connect it to the ocean floor, then you might actually be able to generate power. It's certainly a good idea; one you might want to explore.
I think the only other suggestion I can make is to locate the lab somewhere warm(ish). This would be logical for a tourism-based thing, although less so for a secret, unknown lab. However, you'd save a fortune on heating (or lack of it).