'Mr.Freud' would interpret your dream like this:

Robust Serpent - Phallic Symbol, this is why it 'stabs an apple' (the apple is relative to Eve, as in Adam & Eve & the Apple in Biblical Christian lore). A 'Robust Serpent' stabbing an 'apple' could be the act ouf intercourse, especially w/ mention of another phallic symbol, the 'sword' and, the bullets from the gun, as sperm perhaps.

Freud has always been too sexual for me. Another interpretation, the ocean or the sea as you referred to it, is always highly representative of emotions going out of control, or feeling highly emotional. Many emotions surround you. Serpents are an interesting creature to see, serpents are symbolic of much - interesting you have a serpent and an apple - sounds much like Adam & Eve (this is something Jung would probably state) - is there some kind of 'sin' in your life? Especially if the serpent not only stabbed it with a sword, but also fired at it with a gun, is there some kind of 'sin' you need to take care of? Something emotional you need to solve? Interestingly your serpent was a man. Are you having man trouble?

If you need more of an explination let me know. Good luck.