The seafloor of our one big Ocean can be separated into distinct basins. You may know the five main ones (Pacific, Atlantic, Indian, Arctic, and Southern Ocean basin) yet this map (Powered by Esri, Source: EMODnet, GEBCO) will show you many small ones, see the map at this link. 

Are you curious to learn more about all the details? You may find reading this webpage interesting: link.  

What happens when we vary the temperature and salinity of water? 

Materials:
-food colouring
-container for water
-barrier to put in middle of container

Method: do two different experiments, one with two different salinities, dyed with two different colours and then move barrier, and the other with two different temperatures
1) 

In this video by NASA get a global view of Thermohaline Circulation. Remember these are not surface currents like we saw with the gyres, rather, this is a movement of water due to difference of temperature and salinity. Look carefully when the video is near Iceland and Greenland, the area of deep water formation. 

Think and discuss what would happen if this circulation stopped.

Play the Go with the Flow game, read "how to play" first, link.

This rising and sinking of water is very important to transfer hot water away from the equator and regulate the overall temperature of our planet.  As you'll see in the game there are two main factors influencing the movement of water:

Temperature: warmer water will rise while colder water will sink

Salinity: water with more salt will sink, while water with less salt will ride on top of the saltier water (this is common where rivers enter the ocean).