Yes, the rare Earth magnets I use are available in at least 2 strengths. I think mine are probably the weaker ones but they are quite strong.
It turns out that the tiny magnet above, 3mm diameter x 1.5mm, is way too weak for the long spear on my 90cm speargun. At best, it might help locate the spear and help keep it there just a little bit. I estimate that it would likely need at least 4x the holding power just to properly help locate and help retain the spear; it would likely need a lot more to replace the spear line crossover, which I suspect would be impractical for my compact muzzle and superlight speargun. It is not my goal to replace the spearline crossover, I don't think it would be practical for my set up. Electo-magnet?!
I could stack several 3mm disk magnets but I also bought some 3mm diameter X 6mm cylinder magnets. So, like 4 disk magnets on top of each other,; see photo of my magnets above. A cylinder magnet will likely be the next thing I try. I suspect it will not be enough. After that, maybe 1 or 2 5mm diameter X 5mm magnets?
Rust, I hadn't thought of that. These are rare earth magnets - do they contain a lot of iron, like traditional magnet?