Firstly it is very important to understand the difference between these large water bodies that are the Sea and Ocean.
By definition it goes this way:
Sea: A sea is a large body of salt water that is surrounded in whole or in part by land. More broadly, “the sea” is the interconnected system of Earth’s salty, oceanic waters—considered as one global ocean or as several principal oceanic divisions.
Ocean: An ocean is a large area of salt water between continents. Oceans are very big and they join smaller seas together. Together, the oceans are like one “ocean”, because all the “oceans” are joined. Oceans (or marine biomes) cover 72% of our planet. The largest ocean is the Pacific Ocean.
Coming back to the question, as far as land is considered we can mark precise boundaries but in the case of waters there cannot be a specific line of boundary that differentiates two distinct water bodies.

When the sea reaches to an opening where it is no more surrounded by land on most of the sides and merges with the large water bodies that we call ocean can be considered the merging point.
Thebasic differences in the two would be as followed:
• Sea is smaller in size than the ocean.
• Seabed has been explored by experiments but, the ocean bed is too deep to explore.
• Light can reach up to the seabed but, it can’t reach in case of the ocean bed.
• Seas are closer to land surfaces but the oceans are vast and unfathomable.