Hi! Im going to teach you how to make quicksand in minecraft without mods!

1. Dig a 3x3x2 (Hope I got the dimensions right) hole. 
2. Place any type of slabs in the bottem of the hole.
3. Place any block ontop of the slabs, I used sand.
4. Place a boat in the middle of the slab area.
5. Jump on the boat untill it barley sinks in.
6. Get in the boat, you should fall in completly.
7. Shoot the boat
8. You're done! Now go have some fun! :D

Notes: You can use the slabs and blocks above them multiple times, If you mess up steps 5, 6, and 7, just repeat them. (Make sure to remove the old boat!)

If your a visual learner, I made a video on the topic.