The six year age gap between you two is interesting because that maps onto developmental stages. When you were 3-4 seeing the Sandman, he was 9-10 - right at the edge of losing that childhood openness to the unseen.
Our childhood patterns become the exact triggers in the connection later. My twin had abandonment wounds from age 5, and guess what behavior I unconsciously mirrored back to him repeatedly before I understood what was happening? Disappearing when things got intense. The mirroring pulls forward the exact unhealed childhood moments, not just personality traits. Your underwater memory might be important because of how he responds to feeling overwhelmed now. Does he shut down? Go silent? That could be the adult version of what your child-self sensed about how he processes being pulled under by life’s waves.