Research ships deploy instruments called [CTDs](https://en.m.wikipedia.org/wiki/CTD_(instrument)) that collect seawater. There are sensors on the CTD that measure the salinity of the seawater (along with other things of interest). The collected seawater is also measure to calibrate the sensors on the CTD.
Ady42 t1_is2ok2a wrote
Reply to comment by BowwwwBallll in Does the salinity of ocean water increase as depth increases? by rhinotomus
Research ships deploy instruments called [CTDs](https://en.m.wikipedia.org/wiki/CTD_(instrument)) that collect seawater. There are sensors on the CTD that measure the salinity of the seawater (along with other things of interest). The collected seawater is also measure to calibrate the sensors on the CTD.
There are also [ARGO floats](https://en.m.wikipedia.org/wiki/Argo_(oceanography)) that move around the ocean taking regular measurements of the seawater for things such as the salinity.
At a glance the seawater around Hawaii looks a bit saltier than in some other places, but not the saltiest there is.