I'm reasonably sure they're not identically accurate any more, but good enough for the kitchen.
The kg was initially defined by the mass of the water, but as the kg has been redefined it's drifted a tiny bit.
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I guess in this context the more significant factor is that seawater has a slightly higher density than pure water.
Apparently the "new" kg (based on meter and second) is within 30 ppm (parts-per-notation, TIL) of mass of 1 litre of water.
Like I said, good enough for cooking!