Re: Neither one nor the other.
You could dive using rebreathers. Those are compact, allow extended depth and bottom time and commercially available.
In order to dive below 60m (some say 40m) you need special helium mixture, it can't be done with compressed air. Diving to 80m with simple compressed air is medically impossible. Which means that if you need to do it 4 times on 4 sites, you'll need huge amounts of helium tanks to refill the diver's dive tanks. Also, decompression from such depth takes very long and needs helium tanks all the way.
Also, you won't be able to do that with only 2 divers, you'd need at least 2 spare divers remaining on alert for rescue if something goes wrong.
In real-world scenario, you'd have a team of probably 6 divers in a decompression chamber for several days, staying at 80m depth pressure with helium atmosphere, and the decompression chamber would be lowered to working depth where 2 divers would exit and do the stuff, 2 were on alert for emergency, and 2 would be on observation and guidance probably with a small remote-controlled submarine. All this without being noticed by the NATO navy in the neighborhood.