[numberonepal]

...the only torpedo carried by the P-8 is the Mk 54 Lightweight Torpedo as far as we know. Have you seen the Monkeywerx article? Getting the Mk 54 into the vicinity seems totally legit with the HAAWC system. However, he makes no assumptions as to the capability of the Mk 54 in the water.

The Mk 54 is a lightweight torpedo which means it can't carry a huge payload. Reports are that the explosions tripped seismic detectors at 2.0+ on the Richter Scale. Another report from Der Spiegel says High Explosive Devices Equivalent To “500 Kilograms Of TNT” Used To Destroy Nord Stream. Would a little over a half ton of TNT cause a 2.0 reading? Could the Mk 54 house a micro-nuke? Enquiring minds would like to know.

[LonePalm]

The issue is not whether the P-8 could deliver a Mk 54 to the area of the pipeline. It could. Likewise it is not about whether the torpedo's warhead could damage the pipeline. It probably could.

The issue is whether the torpedo could find the pipeline once it was in the water.

There are simpler and more effective ways to damage the pipeline than using an aircraft to deliver a torpedo.

WWG1WGA

Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)

[Keflavik76]

Another possibility is that the P8 used a depth charge set to explode near the bottom of the ocean in close proximity to the pipeline. Check out the armament list. 

Boeing P-8 Poseidon Maritime Patrol Aircraft | Military-Today.com