Seawater temperature is constant or fixed at the surface because of its exchange with atmospheric heat. In fact, the sun's average incoming energy at the earth's surface is four times higher at the equator than at the poles. Moreover, the infrared radiation heat loss average to space is more constant with latitude.
In effect, there is a net input of heat to the earth's surface into the tropical region which means that this is where we can find the warmest surface water. This is evident in the graph shown below since it almost reaches the peak/highest temperature.
On the other hand, the salinity of the surface seawater is primarily controlled by the equilibrium between evaporation and precipitation.
In the tropical or equatorial region, sea surface salinity is also controlled by rainfall and river overspill; these sources of water phenomenally regulate how oceans interact with the atmosphere.
As seen from the graph above, at 0-degree latitude, the salinity is low compared to the other latitudes. This is because of the rain from rising atmospheric circulation. This is further supported by the fact that its geographic location is primarily where rainfall and storms normally occur.
