As someone with diving experience I laughed at parent comment's "anyone with scuba gear" assertion too - a classic example of "I'm an expert at everything because I'm a smart programmer who makes $200k a year" arrogance that is endemic here - but you're right next to them.
- The timing of the explosions doesn't require "multiple coordinated teams." It requires timers. Or, as is common with naval mines, an acoustic arming/triggering signal.
- Commercial diving happens several times deeper than 80m. Hundreds of meters, in fact.
- Saturation diving techniques mean a crew could be brought to the working depth inside the ship's chamber, transferred to a diving bell, dropped down to the pipeline, do their work, go back into the bell, be brought up into the ship, transferred into the ship's decompression chamber, and decompress.
- You're assuming such an operation would be based from the surface. If a nation-state was involved, who says they arrived or departed by surface ship? The Jimmy Carter is for exactly this sort of stuff. It was practically a sport for the USSR and US to tap undersea communications cables. We had the capability to do that without being undetected (including from systems that are designed to detect faults and tampering, but also deal with the cable armoring, as well as avoid very high voltages used to power in-line amplifiers) more than fifty years ago...but you think slapping some explosives on the side of a pipe is technically challenging to a nation-state, or even a commercial diving company that does oilfield work?
- You assume that explosives were planted by divers, and not, say, a submersible (see: Jimmy Carter) or ROV. In fact, you're assuming explosives were planted, and someone didn't use a smart torpedo of some sort.
One thing, though: I agree with others that it seems unlikely Russia would do this to themselves when, you know, they can just not supply gas into the pipeline.
One possibility: after Russia threatened using nuclear devices, both pipelines were blown up was either done to send a message - or as direct punishment for the fairly unprecedented threat to use nuclear weapons.
It also has the side effect of taking any 'heat' off EU politicians to cave if energy prices soar and/or if people freeze in their homes. Russian gas is for the moment very much off the table. If those pipelines can be repaired, it won't happen overnight, or cheaply.