Barit1
I suspect that as it drifts into water of varying density (warmer, cooler, salt content...) this will affect its buoyancy.
That only works with objects that are as compressable as water (or less). Many such objects exist, they are almost all made almost entierly of water! A closed cell composite will be many times more compressable than water and will increase in relitive density as it sinks. yes its buoyancy will change in tempature salinity if it is buoyant. Once it starts to sink it will go to the bottom increasing in relitive density as it goes. =