If it was just swimming in the sea, it wouldn't be a problem for Lin Yi. He only needed to circulate his Qi to keep his body warm, and the Jade space had an endless supply of Qi as well as oxygen. Lin Yi could stay in the sea for ten days and a half a night.
However, since the sea water couldn't stop Lin Yi, it naturally couldn't stop the evil cultivator giant, West Mountain old sect. It was impossible to escape from West Mountain old sect by just hiding in the sea. There had to be a limit to how much one could deceive themselves.
Moreover, the sea was different from the ground. Even if they could deal with the sea, they still had to face all kinds of unpredictable dangers, such as sea beasts.