Is it easier for organisms to live on land or in the sea?

This is a question I commonly use when giving interview practise for potential Oxbridge students. A question like this doesn't have to have a corrrect answer necessarily, but should evoke a discussion and an evaluation of the different factors that must be considered. For example, students may look to discuss the impact that salinity has in the sea and how this results in salt-water organisms requiring more specialised symptoms to maintain osmolarity. This question uses the word 'easily' and when answering this myself I would always ensure to define exactly what it is that is 'easier'. For example, when considering respiration, it would be 'easier' to live on land as it has a more easily accessible source of oxygen, but with regard to thermoregulation, living in a body of water gives a more constant temperature as water is traditionally difficult to heat up.