Another illustration of using the water level
If you haven't read the basic water level explanation you need to read this first

Paul wrote:

My task is to level a 20' long ledger board for the deck. From your 
drawing, it looks like I would take two 10+ foot tubes, insert one end 
of both tubes into a bucket of water.

There's only one 20 foot tube (mine's more like 25'-30').  The dashed line is not a second tube.  It symbolizes putting the tube in a second location.

Then, I would siphon (i.e. pretend like the tube is a straw and draw the
water up to my mouth) each tube? 

Yes, I suck water into the tube,  If you do it just right, once water has come up and over the edge of the bucket and back down the outside you can lay the tube near ground level and it will fill itself.  It doesn't matter how you fill the tube, just be sure there are no bubbles large enough to break the continuous run of water.

Maybe a Second Illustration Will Help

Make sure the bucket is sitting on something such that the surface of the water in the bucket is roughly at the height of the ledger board ( can be off several inches ).  Put one end of the tube in the bucket and make sure it stays submerged.  Fill the tube with water.  Get a stick (I normally use a yard stick) and attach/tie/tape the stick to the tube such that the water level in the tube falls is near the middle of the stick.

Hold the stick up to one end of the ledger.  Mark the top of the ledger on the stick or tube (ledger mark).  Without moving the stick, also mark the water level (water line mark).  Remember which mark is the water line mark and which is the ledger mark, maybe mark them with different colors.  Move to the other end of the ledger.  Move the stick up and down until the water level in the tube lines up with the water line mark. The ledger should line up with the ledger mark.

The stick isn't absolutely necessary; you can make your marks on the tube. But, the stick it makes it easier to handle the tube and keep it reasonably straight and vertical.  Also, if you intend to reuse the tube you can replace the stick.

jim