I have one big one, pointed tip around my ankle that is made of titanium or something, and a small one on my BCD that is blunt nose, that is stainless steel (but still gets some rust stains on the blade... but, it still works). reasoning for two knives is you never know if you might get tangled up or something, so one might be easier to reach than the other. having one blunt and one sharp is good depending on the occasion you need to use it for.
I got the knives in 2014 from amazon, i don't remember if I paid this much for it back then, but, this knife has been holding up well and hasn't had any rust stains on it: https://www.amazon.com/gp/product/B00362TXRU/ref=oh_aui_detailpage_o06_s00?ie=UTF8&th=1&psc=1
I got this one at the same time for my BCD, and I love the size, but it does get some rust stains on the blade, especially where the writing is, there is a way to clean it off, but i haven't try too as you need to do something special so you don't damage the blade or something: https://www.amazon.com/gp/product/B004RDVG22/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1
overall the ankle knife has held up the best (in terms of not getting rusty). The BCD one has come in handy as well, but if i get a new one, i will try to get something that is titanium or actually stainless steel, as I don't believe this one was... or maybe it wasn't marine grade stainless steel...
anyway, i haven't had too many situations where i needed to use my knife underwater, but it is better to have it and not need it, than to need it and not have it. I've cleaned up a lot of fishing line with these.
hope that helps!