A tide gauge is a type of instrument which is put into the ocean to measure the weight of the water on top of it. Therefore, if the weight is heavy, there is more water, and thus, the wave is higher. You could take a boat to your research site and measure the temperature with a thermometer. If you want measurements at that spot for a long time, you could put a buoy there. You could take a lot of measurements from ships.
Satellites can orbit the Earth several times in a day.
By placing instruments on a satellite, an oceanographer can obtain data from all over the world in a short amount of time. These instruments are able to measure the temperature of the ocean's surface, the height of the water, the speed of the wind above the water and many other things.