How do I get items after they fall into the water?

You currently can't dive in Zelda. So if something falls into the water and you can't pick it up while swimming over it you are out of luck.

The only solution how you interact with items below the water surface is the magnesis tool. So you can try to drop a magnetic weapon (or search for something magnetic) and try to "fish" with it, to push it to the beach and pick it up from there.

Try stasis.

That's what I did. I tried cryonis but there was too much of a gap between the mountain slope and the ice pillar; obvs I put it as close as I could. But stasis worked finally.

I created a cryonis slab near the underwater gems (but not TOO close or you can't see them or hit them), then used stasis on them one at a time and hit them with arrows to project them into shallower water so I could pick them up. A Lynel bow or Duplex bow works great because you get more hits with less arrows. Also you can pick up your arrows when you're done, as they float to the surface! Hope this helps!

Also if you jump into the water from high enough he does a small dive animation if your quick and dive right onto it can get down to grab some things, doesn't get you in real deep tho

Had this problem, diamond fell into a lake, what I did was stasis it and use arrows to hit it to shore. I tried to use magnesia and fish it out but it was very hard, I like this method better, it was how I did it. If it is too far use cryonis, bombs did not work for me though. Don't worry about your arrows, they will float back up (unless you are not using normal arrows or are using a x2, x3, or x5 bow).

I made an ice pillar beside the gems that fell underwater, and jumped off wearing zora armour so link dives below the surface momentarily and you can grab whatever fell