[Salamat]

I visited abkhazia and was told they have wood that is very hard it does not float in water. Could not verify that.

[eco]

Yeah, Lignum Vitae doesn't float. This Old House has a clip showing this off. https://www.youtube.com/watch?v=CI39sMIy6mE

[Someone]

Nitpick: they likely are highly correlated, but hard woods do not have to have high density.

Looking at https://www.wood-database.com/wood-articles/top-ten-heaviest..., the entire top 10 is heavier than water (#10 is almost 20% heavier; #1 over 35%)