I doubt if this dream means that you really want to have sex with your brother.

Since you and he are probably quite similar, being siblings, I think that he may represent an aspect of you. Since you are focusing on his "male part", perhaps in your dream he represents the side of you that is traditionally "masculine" - assertive, rational, intelligent, practical, non-emotional.

On the other hand, bodies of water such as the ocean are feminine symbols, representing your emotions, your instincts, and your unconscious, while a journey by ship can be a sign that you are exploring your emotions.

Burying expensive things at the beach could represent "burying" negative thoughts and emotions. You are now thinking about "digging them up" but are stopped by the side of you that wants to avoid acknowledging emotions (represented by your brother).

Note that the person who catches you having sex with your brother (becoming less connected with your emotions) is a woman (your mother.)