There's actually some very accessible Magnetite spawns near the starting area, but you've got to go through a cave system that goes straight down to find it. It's in the Jellyshroom Caves, and you can usually go as far down as your Seamoth allows before it starts to take damage, and then swim around and collect it, going back to the Seamoth to replenish your oxygen when needed.
There's a really easy entrance location, very close to where my lifepod spawned in my game, but I can't find any videos on that specific location. There are a load of other videos on YouTube showing several other entrances, but that's only if you want to spoil stuff