Insanity
Even if he might not be planning on swimming, he walked into the sea right up until his knees were submerged, and began resisting the assimilation. It was not an easy task, and was the only experience he ever encountered that he couldn't build a resistance to by cultivating in this condition.
Maybe he needed to meet some criteria to start building up a resistance to it. But until he figured that out, he would just train manually. Lex managed to resist the sea for several hours before his energy began to run low, so he took a break.
After saying farewell to Nemo, he teleported back to his meditation room and began to recover while sitting atop the dragon's head.