They wouldn't have needed anything approaching several hundred kilos of explosives to breach a pressurized pipeline.
Did you read the WSJ article linked above?
"Armed only with diving equipment, satellite navigation, a portable sonar and open-source maps of the seabed charting the position of the pipelines, the crew set out. The four divers worked in pairs, according to people familiar with the German investigation. Operating in pitch-dark, icy waters, they handled a powerful explosive known as HMX that was wired to timer-controlled detonators. A small amount of the light explosive would be sufficient to rip open the high-pressure pipes."
Traces of the HMX were found on the boat.
Six people, including 4 with rebreathers, and enough supplies for several dives, would certainly fit on a 50' (15m) cruising sailboat. This is the model that was allegedly used:
BAVARIA C50 Interior | BAVARIA YACHTS