Blade of the Mer-men
Aside from the blue colour chosen for this blade, there are 4 significant points of interest here which make it obvious that this is a sword of the sea-people:
1. The decorative fish-head pommel. Not a real fish head (which would rot away pretty quickly, leaving the blade unbalanced), but carved whalebone. Its teeth are sharp, making it a perfect secondary weapon against smaller fish should they get too close.
2. The “flotsam” crossguard. Made from a piece of driftwood, possibly from the wreck of a cursed ship. In fact yeah the mermen make their crossguards from ships which they have sunk, casting the crew to the bottomless depths of their kingdom. In order to make their first crossguards they attacked that ship with tridents and nets.
3. The “fishbone” fuller. Unlike the pommel, this is real fish-spine. Each mer-lad has to kill a fish as a rite of passage, and then they use its spine as a fuller. This gives the blade the same flexibility as a spine, which is a good thing. The bigger the fish they slay, the longer the blade. One mer-hero of yesteryear, Seabass Pacific, slew a whale and so his blade was over 30 metres long and weighed six tons. He tragically drowned on his first assault on a fishing vessel. So did Trout Black (named after the Black Sea) who killed a jellyfish.
4. The Seaweed tip: Makes it all sea-like.
Also of note is the slight curve in the blade. This isn’t accidental, rather drawn to represent the gentle swaying of sea-plants in the current. It also helps make the sword more deadly.
I’ve yet to swing a sword underwater, but I actually think it would be quite hard to do so with enough force and speed to harm anything. For this reason the mermen can also spit acid, and use this as their primary attack, and only really use their swords to stab hulls of ships.