Weight gain

While Lex had misunderstood the situation, assuming that the pressure he felt was not real, Mateo knew better. Without the suit he was wearing he would have died under that pressure. But as surprising as Lex's situation was, Mateo too was not someone ordinary. His experiences and background afforded him an understanding and comprehension that few had.

He recalled Lex's many abilities and affinities, and immediately joined the dots. Lex had an unusual affinity for Laws. Not any one specific law, or multiple laws, but just laws in general. He could physically touch that which had no physical form, and influence that which defined the fabric of reality.

Given this, it was not surprising that the gravity and pressure laws in the liquid could not hold Lex down. After all, his body was naturally affecting the very laws holding him down. He could just move them aside, though technically that is not how it worked.