Why is Northern ocean water dark blue and Southern ocean water crystal clear turquoise blue?
Two points to realise: 1) What you say is only true for the Northern Hemisphere. Excatly the opposite is true in the Southern Hemisphere, where southerly waters are dark and northerly waters are clear. 2) The Southern Ocean is a specific body of water surrounding Antarctica. Southern Ocean waters are usually dark blue. The phenomenon you notice is because in the Northern Hemisphere northern waters are colder than southern waters and cold waters are usually nutrient deficient. Wanna know why? It’s because dead animals and plants sink in water. That might seem obvious, but it means that any nutrients in sea water are rapidly used up by animals and plants and when those organisms die they sink to the bottom taking the nutrients with them. The only way those lost nutrients can be replaced is if water form the ocean bottom, where all those dead organisms go, can be moved to the surface again. That cycling can’t happen in warmer regions because the surface water is warm and warm water floats
