The Big Dictionary of Dreams

by Martha Clarke
It is associated with sea monsters.

It is a sinister enemy that comes to meet you. It represents insurmountable obstacles in your way that cause total fear. It also denotes a state of stalemate. In this sense, it relates to becoming petrified, unable to move on or grow.

Dream Symbols and Analysis

by DreamForth
To dream of a reef suggests you are repressing your emotions. You are preventing unconscious thoughts from manifesting in your life.

The Dream Books Symbols

by Betty Bethards
Barrier to growth; caution. Also, protection while swimming in emotional waters.