The ocean and the sea are also a symbol of the feminine principle, so a dream of being overwhelmed by a wave may suggest that you are being smothered or dominated by a woman in your life, whilst calmly floating in the sea or surfing the waves suggests a good relationship with yourself, and perhaps with a woman.
If you are struggling against the tide, this is a sign that you are not making the progress you had anticipated, but that you are nonetheless gaining strength and wisdom from the struggle.
If a sea monster emerges from the depths, could this symbolize unconscious fear or even an aspect of yourself that is being repressed in waking life?