So I found a big bar of shiny metal and made a set of tongs from it. I noticed that the tongs are non-magnetic and I don't have an issue dropping them I water when hot and they've been whacked by a hammer more times that I'd care to say. A neighbor (sheet metal worker) said it was probably low-grade stainless. What is good to make from that? I still have about 3' of it left and it's 3/4".

The 300 series stainlesses are typically non-magnetic but I would not characterize those as "low-grade". They probably would make good tongs.

In the US standards there are 200, 300, 400, and a few named series in addition to the numbered...

Well, the tongs are working great and were a huge pain to shape - that should've been my first clue.

300 SS is non-magnetic except when work hardened. Not as magnetic as mild but will attract a magnet. 400 is magnetic, stronger than 300 but still not as strongly magnetic as mild. 300 SS won't termally harden, in fact to soften one heats and quenches. 400 will quench harden.

Other magnetic stainless steels are 17-4, 15-5, 18-8, we use these at work. 440 will stick to a magnet pretty good. Remember to be a steel it will have iron in it. The original name was Stains-less since it rusted less than regular steel.

So I'm n hawaii, would his stuff be alright to make a dive knife out of? I'm not looking for anything to shave under water with, but could I make a few knives for my buddies so they have something to swim with while spear fishing? I guess if it won't harden then it would make a pretty crappy blade, but if it only needs to pierce it could work. . . .

I guess I'll let you guys know how they turn out.

Titanium doesn't make a good knife but it is used for dive knives. Stainless will rust. You will need to passivate it to remove the surface iron to increase its rust resistance.

The original name for Stainless was INOX as it was invented in Europe, mostly in Germany.