Low-Tech Weapons and Tools Underwater

[Icelander]

After one of the PCs in my fantasy captain encountered Baron Ak'Zuma, the local lord of the fearsome sahuagin, also known as sea devils or shark-men, he wanted to know what TL2 to TL4+1 materials would make for the best tools and weapons to trade to a race who lives underwater, in salt water.

Coral spearheads or knives may be sharp, but they are more or less single use. Bone spearheads, weapons or tools are sub-par. Magical enchantment can easily produce waterproof tools and weapons that are equal or even superior in durability and strength as steel, but is very expensive.

What non-magical methods are available to surface dwellers to produce tools and weapons that are worth trading to undersea races?

Metals will rust or corrode over time. Granted, even if a metal spearhead corrodes or rusts into uselessless within a year of hard use, that's still a an improvement over a coral spearhead that breaks the first time you hit bone while killing a whale or sea serpent.

What metals provide the optimum balance between corroding or rusting slowly underwater and having desirable properties compared to bone or coral?

Is there a meaningful difference between the durability of copper, brass or armour- (or weapon-) grade bronze under water? Does steel last longer than bronze or does rust ruin items faster than corrosion? Is steel better or worse than wrought iron?

Are there non-magical alloys possible with TL4 technology that would be worth the extra trouble of making them, in terms of durability and quality?

What about TL4+1 metallurgy, provided with a dormant volcano enchanted with a system for trapping magical fire, melting metals, mixing them and purifying? Research might yield TL4+2 metallurgy from that magic.

How hard is it to make stainless steel if you have the Measurement spell, high skill at Metallurgy/TL4+1 and a way to melt iron down to liquid form?

Assume that sahuagin live at depths of 100' to 1,500' in a subtropical salt ocean, generally fairly close to shore. It might be possible to store tools and weapons between uses under slightly difference conditions, if it is worth it, but the lifestyle of sahuagin is generally very mobile, with males hunting, raiding or patrolling almost constantly while they are awake.