It wouldn't work on a CR 23 Kraken (MM 193) per se due to the size restriction, but maybe something like a Juvenile Kraken (GoS 239) from Ghosts of Saltmarsh would be susceptible to it.
The wording:
Adjust Density
At 2nd level, as an action, you can magically alter the weight of one object or creature you can see within 30 feet of you. The object or creature must be Large or smaller. The target's weight is halved or doubled for up to 1 minute or until your concentration ends (as if you were concentrating on a spell).
While the weight of a creature is halved by this effect, the creature's speed increases by 10 feet, it can jump twice as far as normal, and it has disadvantage on Strength checks and Strength saving throws. While the weight of a creature is doubled by this effect, the creature's speed is reduced by 10 feet, and it has advantage on Strength checks and Strength saving throws.
Upon reaching 10th level in this class, you can target an object or a creature that is Huge or smaller.
There's no save, it just halves or doubles the density of the kraken, so if it attacked your boat, you could double its density to make it sink to the bottom, or if you were fighting it underwater you could half it, to make it rise to the surface allowing you to recoup for 1 minute.
There's also no limit on its uses and its range is 30ft, so you could just hold an action in order to hit it before it got close enough.
Additionally, you could murder most NPC/humanoid characters by adjusting their density in water and having them sink like a rock to the bottom of an ocean, wherein they would have to swim hundreds if not thousands of feet straight up before running out of air (unlikely).
Overall, it feels like this Adjust Density ability is very strong in water and has an insane amount of potentially overlooked utility for a 2nd level ability, with no limits on uses, and no save.
Best Answer
When there are no rules, we fall back to DM rulings
Clearly if your mass is doubled you will find swimming to be more difficult than normal. However this situation is not covered anywhere in the rules, so DMs are expected to make reasonable rulings that respect the rules and align with realistic expectations.
When something becomes more difficult, we usually deal with it by using disadvantage. The rules give the example of making a Strength (Athletics) check to swim in rough water, a reasonable analogue would be to allow the kraken to make a check with disadvantage to swim with their density doubled.
Wildemount Content is not balanced
Explorer's Guide to Wildemount is a source book not written wholly by WotC. Although they did support its development there have been significant issues with the quality of this book. Particularly, players raised doubts that there had been anywhere near enough playtesting - especially compared to the usual high quality of source books put out by WotC.
Much of the book is broken, to put it simply. Echo Knight is the worst offender by far, but there are many other problems.
As a DM, I would encourage you to approach EGtW content the same way you would homebrew. It's unstable content that will need you to put in work to balance it, but it's also optional so you can opt to not allow it at all.