Water - symbolizes life and emotions.

Blue - is a cool and 'analytical' color, and there are many representations...
Blue has always something to do with intellectual and spiritual events, insights, experiences, and mental processes.  Blue is a symbol of emotional contentment that we all would like to attain. It indicates that you want to live wisely and with compassion. Blue stands for intellectual goals, insight, faith, spiritual maturity, enlightenment, relief from worry, and assistance from outside sources.

Dark blue - depression, negative thoughts, and aggression.
Light blue - the sense of intuition, having a wide awareness of life, religious feelings, and the 'coolness' of nature.
Soft gentle blue - a typically feminine color, representing the nurturing and emotional expression.
* Blue may sometimes symbolize the universal or collective unconscious (as distinct from the individual unconscious). Perhaps the dream is asking you to base your life on intuitions that come from a deep source within your psyche.
* Alternatively, blue may represent the power of the conscious mind, particularly if it is the blue of the sky.
* The blue sea may symbolize the unconscious or the feminine (anima, mother, or Great Mother, the wise self).
* At an advanced level of mystical awareness, blue may represent the primal energy from which the universal life-force comes.

Sand - represents lack of security, changing or 'shifting' situations, passing of time, lack of emotion or emotional 'nourishment', and irritations.

Beach - symbolizes family gatherings, relaxation, possibilities, intamacy, and pleasure. A state of mind that is open to verbal, and non-verbal experiences, intuitions, emotions, and links between the consciousness and everyday life.
It also represents a boundary or, 'threshold', between our individual self and effort...our potential in the universal processes of life and death, also a barrier that 'fear', or the lack of confidence creates.
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Have you had an argument, or discussion with your boyfriend recently on where your relationship is headed?
Are you both in agreement on your goals or 'needs' within your relationship?

You are both probably connected on a 'spiritual' level, and very much 'in tune' with the needs and desires of the other.
If you've had a conversation recently on where your relationship is...or even separately you both have been having 'feelings'...
(are you getting more serious and possibly thinking of marriage, or is it cooling off and you're both feeling that you're losing touch with each other and having doubts about the relationship)
...it could cause you both to have very similar dreams.

I hope this gives you some insight. Best wishes!

Glad I could help. Good luck with whatever you both decide to do!