Salinity is the measurement of how salty the water is.
You will probably find that salinity is nearly the same at the three spots you measured. Scientists measure salinity in parts per thousand (ppt).
You will see that the measurements only range from 32 ppt to 37.5 ppt.
That means out of 1000 grams of water, the salt content only varies by about 5 grams. (1gram = the weight of a paper clip)
There are three main factors that affect salinity:
When it rains, the water dissolves minerals and salts from the rock on the earth’s surface and washes them into the ocean. Minerals and gasses from volcanoes that erupt underwater also contain salts that contribute to the salinity of the ocean.
It might be less salty at a river mouth where a lot of fresh water empties into the ocean.
The Dead Sea in the country of Jordan contains the saltiest water in the world; 300 grams of salt per kilogram of sea water. That is ten times as salty as the ocean!
5 cups of water to two tablespoons of salt (approximately 2.5% salt).