Water is compressible; it has a bulk modulus of about 2.2 GPa. So at the 1086 bar at the bottom of the Mariana trench (~109 MPa), it'll have compressed about (109 / 2200) ~= 5%. Materials with a different bulk modulus to water may start to float at sufficiently high depths.

https://en.wikipedia.org/wiki/Bulk_modulus#Selected_values