Graham Le Compte of Christchurch asks :-

Why am I more buoyant when floating in sea water at Tonga than around New Zealand?

Pat Langhorne, a physicist at Otago University, responded.

When we float in water we do so because a buoyant force acts upwards on us, in the opposite direction from gravity, which pulls us down. The buoyant force depends on two things: the volume of the submerged portion of the floating object and the density, or mass per unit volume, of the liquid in which the object floats. So when we float in the sea, the amount we sink is determined by how dense the water is: we are submerged less, or float higher, in water that has a high density than water that has a low density.

The density of seawater at the ocean surface depends on temperature and the saltiness or salinity of the water. If you look at a map of surface densities over the world's oceans you will find that the annual average density of the waters around New Zealand is 1026 kg per cubic metre, while around Tonga this figure is 1024 kg per cubic metre.

The average surface salinity of the water around Tonga is higher (tending to make the water denser), but so is the temperature (making the water less dense). The overall result is that the water is less dense around Tonga than around New Zealand, and you would therefore expect to sink deeper into Tongan waters. However, if you are in a shallow, sheltered lagoon in Tonga then evaporation could make the water of the lagoon even saltier, causing the water there to be denser. In a salty lagoon you would float higher in the water. Then again, maybe your feeling of being more buoyant in Tonga is psychological because you're on holiday and the weather is nice.

Incidentally, note that the buoyant force is the same on you in New Zealand and in Tonga and is precisely equal to your weight if you are not moving in the water. This force is kept the same in waters of different densities by adjusting the amount submerged.