Until just last week, I used to use cheap plastic water bottles (bottled water you buy in any supermarket). Getting water was easy. You squeeze the bottle, then you place it in the pool of water — even if just a few inches deep. You then poke the bottle here and there to get it to pop back to normal shape, and in the process, water is sucked into the bottle (say about 1/3 full). You repeat the process (squeeze bottle to get the air out, then place bottle in water, then get the bottle to pop back into shape) 2 or 3 times, and you have a completely full bottle. Oh, and you prefilter simply by rubber banding a very fine mesh over the mouth of the bottle.
Now that I have switched to bladders, the above won’t work. As far as I can tell, bladders work best if there is a vertical flow of water. Otherwise, and esp. if the pool is very shallow, you may have to ladle the water with a cup.