How to prevent water from freezing

Only full water blocks will freeze. However, flowing water will still hydrate nearby crops. So you can place a single block of water at one end of your trench, and it will hydrate all your crops, and it will never freeze because of the torch.

A pumpkin torch placed under a water block will also prevent it from freezing.

An added benefit of this system is that any items which fall in the water will be swept to one end for easy collection.

Taken to the logical conclusion you get this:

Only the four full water blocks (two in the middle of each row) can possibly freeze, and the pumpkin torches underneath them prevent that. The normal torches are only to prevent monsters spawning. The stone paths prevent animal spawns and are sunken so that you can wall-strafe to quickly harvest without accidentally walking on the crops.

This isn't a space-efficient or material-efficient farm layout, but once built it is very time-efficient and low-maintenance.

No, ice is unable to hydrate crops. I suggest placing torches directly next to the water to melt it, all artificial sources of light can melt ice.

if you have the resources, you can put glass under the water and torches underneath the glass. you can also just use glowstone under the water. It will look nicer than placing torches everywhere

A good idea is to place some torches, glowstone, Jackolanterns, or the 1.8 block, sea lanterns, directly next to the ice. Any form of light will work, except the sunlight when it is day time. By the way, if the only way to get water is far away, flowing water can also hydrate the crops, so using only 1-2 water buckets is fine. Crops CAN grow with no water, but you have to be quick placing the crops on dry farmland, since it will die out very fast. Hope this helped :)

Try to put some ice blocks above the water,because blocks of water below the surface water won't freeze.