Some fairly straightforward physics is your friend here:
To determine if something will sink or float in a given medium, you need to know its density. If the density of the object is less than the density of the medium it will float.
Standard pool water would have a density around 1g/cm3, salt water is higher (depends on the amount of salt in the water).
To calculate the density of the LEGO elements, you need to weigh it, and divide that by its volume.
The slight complication is as Mr Shiny and New states: Bricks can trap some air inside them which reduce their density while the air remains trapped in the brick. Items like the motorbike are almost solid ABS, and so are much more dense than a minifig with a hollow body, arms and legs.