Most hand-to-hand weapons are already streamlined for the obvious reason of minimizing air resistance. But would the same design work for underwater warfare?
My story has a character who is exceptionally skilled in fighting in an underwater environment; he has extensively trained underwater and has the ability to control large bodies of water. He is incredibly tall and muscled. He is strong enough to swing swords and other melee weapons despite the water resistance and his signature strategy when fighting is to create a room-sized body of water around his enemies so he has the advantage due to his training. However I am not sure what the design for his personal weapon should be that could be effective in both on land and underwater combat. Would a typical sword work? or would you need a weapon specifically suited to cut through water rather than air.
I also am not sure what material would be best as it would need to be rust proof.
I have an idea which I will put as an answer and feel free to give your thoughts on that design.