200 feet is doable by a military diver no problem. I'm a USMC Navy diver. I was Recon but the Navy runs the dive school at K bay in Hawaii. (at least they did 35 years ago 1986) yes you would have to decompress.
What happens is that you are breathing compressed air. The nitrogen in the compressed air gets in your muscles and your blood stream. If you surface too quickly the gas expands and you get the bends (technically decompression sickness). The process of decompression is to stop at regular intervals and allow time for those gasses to be metabolized and replaced with less compressed gas as you ascend.
200 feet down in the Baltic is colder than a well diggers a$$. Would require a dry suit and major decomp time. setting those types of explosives takes bottom time and the more bottom time the longer to decomp.
I think it's more likely done from a sub, but who knows? With a sub the divers are locked out of a torpedo tube (olden days) now they use a lockout trunk which is a special room that can be flooded and then opened to the outside. Diver gets in the tube, tube is flooded and the outer hatch opened. Later the process can be reversed.
That’s pretty much what she said about the deep diving. She said that if you are looking for something 200-300 feet deep, or more, like a pipeline, the norm is to use a small sub or a remote vehicle of some type. ROV? She said they don’t require decompression time coming up and can be operated a good distance away from the target.