Sadly I know nothing about sea witchcraft, I assumed there was lore surrounding your local area and perhaps traditions handed down to you. In my own area we have no sea but local lore surrounding the river as well as a deity associated with it and traditions passed down. That is where much of traditional witchcraft practices begin.
Others make up their own path as they go along, walking a crooked path so to speak, trying new things, experimenting and creating their own tradition.
I think we need to get rid of labels myself, witchcraft is witchcraft. It is the craft of the wise, it is the knowledge of herbs, and the desire to gain knowledge of the universe, both physical, and non-physical. Most of all, it is that inner burning desire simply to know. It is the craft of creating change both within and outside of one's self and it's specific methods vary from person to person.
Your first step would be to gain some connection with the energies of the sea, that could be researching various deities and mythologies, sitting and feeling the energies of the sea, researching local myths, and finding out how the sea can aid you in spiritual development.
Others may have a different opinion, but thats witchcraft!
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