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They hate Russia and are not on very good terms with Germany (in particular, they ask for reparations resulting from WWII). There have been two technical possibilities:
A Polish vessel could have chained down a large number of explosives with precise and synchronized clocks across the direction of the pipeline. Though no single explosive charge could be positioned precisely this way, given a sufficient amount of charges, at least one of the charges will inevitably be positioned close enough to the pipeline. Recall that water is incompressible; it acts like a hammer even if the explosion is nearby.
Alternatively, Poland could have employed deep divers up to 100 m (https://en.wikipedia.org/wiki/Deep_diving) and few explosive charges. This would require preparation, equipment (e.g., decompression chamber), calm waters, and time and is within the bounds of technical diving.
Both are absolutely within the realm of what Poland could do even using even a commercial ship, e.g., a large fishing vessel. In fact, one of the Polish vessel was near the site at the time.
What we do not know is whether the Poles did this themselves or whether they let the Ukrainians do the dirty job using a Polish vessel.
expressed the wish that the pipelines be not used;
promised to take them down (e.g., cf. the speech of Joe Biden to destroy the pipeline earlier this year);
had the capability (the US military vessels are present in the sea; e.g., cf. Was USS Kearsarge LHD-3 near the explosion sites of Nord Streams shortly before the explosion time?);
were even the first ones to warn the Germans that an attack is planned and
profit from it (they can now sell more LPG at higher prices to Germany and have cut Russia from is export capabilities for years if not decades).
According to criminal-investigation standards (i.e., if the USA were a person), in the absence of direct evidence, it wouldn't even matter whether USA really did it because the above (desire, capability, presence nearby, and no alibi) would be enough at least to accuse, perhaps even to prosecute USA.