Well, they can't exactly test that particular system without causing massive ecosystem damage. As for your first comment - you are commenting from a politically motivated position, not a technical one. Said another way, you are making the warfare 101, chapter 1, paragraph 1 mistake.
The 2011 Tohoku earthquake that created the devastating tsunami was an energy release of something like 600 million Hiroshima bombs. Nuclear weapons are just the wrong order of magnitude to make a tsunami, this isn't about politics, it's about scale.
What every nation, Russia included, talks about and shows off has very little in common with what they can actually do.
In Russia's case we mere civilians can just directly observe the difference a lot of the time, enough for me to be comfortable projecting the same assumption where only real military analysis could actually tell for sure. The claim that this weapon could cause a tsunami is within the set of things even civilians can disprove.
This option is not available to us civilians for more competent cases such as China or Iran.
If you mean this, it's mostly (and certainly the tsunami part is merely) propaganda nonsense: https://en.wikipedia.org/wiki/Status-6_Oceanic_Multipurpose_...