A bowline knot is an excellent rescue knot as it tightens when under load. However, its very use as a rescue knot may make it hard to tie - for example, if a person fell and damaged one arm to the point where he or she was unable to use it to help tie a knot!
How can I tie a bowline knot with only one hand?
Yes it is possible. There are many methods, but here is one I found by googling "single handed bowline knot" (first result): http://www.animatedknots.com/bowlineonehand/index.php
With the rope round your back, hold about half a meter of the short end in your hand. Hook the standing end with your thumb to form a loop around your hand. Pass the short end round the standing end and pull it back through the loop. Secure the end with an overhand knot to the loop.
My favorite way to tie a bowline is a little unorthodox, but it's easier, especially 1-handed.
Make a slipknot in the standing part.
Grab the standing part.
Twist to make a loop.
Reach through the loop and grab the standing part.
Pull the standing part through the loop.
Pass the working end through the slipknot.
Capsize the slipknot.
Now you have a bowline. It may be left- for right-handed, depending on the direction you did step #2. One is slightly more reliable, but both are good knots. You can improve the reliability by hitching the loose end to the loop. For critical (human) loads, and in slippery (synthetic) line, that hitch is more important.
Be sure to practice this a few times before you need it. The slipknot needs to be snugged up but not tight. You need to know where to put the slipknot to get a big enough loop at the end.
One way this approach is really useful is when you will need to hitch to something very quickly. You can prepare the slipknot ahead of time. When the time comes, pass the working end around the object, through the slipknot, and capsize.
Shawn has already answered the question, but I would like to add that it whilst it may be useful in some situations, it can be a bad habit to get into, and dangerous in certain situations.
As you can see in this video - http://www.youtube.com/watch?v=auj4T1PYSRI - it involves getting your arm very tangled into the rope. If the rope was to suddenly be pulled, your hand would be done for.
By all means learn it, but try not to use it!
