= The manual way =

You can let sand fall into the water to split it into columns (7x7 should be good) and then drain it with sponges.

It should probably be the easiest to go into the water, place one sponge on the sand wall, break it, place the next one lower down etc. while you go down with the water. Then you use an ender pearl, elytra or pillaring to get back up and repeat it with the next column.  
Alternatively you could also drain the top, then break the top of the sand walls, then drain more, etc., but getting the sponges back would be more difficult that way.

At the end you can break the sand either with a good shovel (instamining) or by breaking the lowest block and quickly placing a torch in its place.

[Here](https://www.youtube.com/watch?v=jYQfF3k931w) is a video about it.

= The automatic way =

Since you're apparently a person that likes farms, this might be more your thing: You can build a flying machine and item dispensing system to automatically fill everything with sand. This works best in a rectangular area.

Since it would be quite difficult to explain the whole flying machine in text and pictures, I'll just link a video: https://www.youtube.com/watch?v=T1UGBmfLTII