It's also possible to loosen that retained tip up enough to remove it by putting the snake in a container (deli cup or whatever is appropriate for the size of the snake) with some seriously damp sphagnum moss. Paper towels or newspapers are supposed to work as well, but I've personally had luck with the moss. Be sure to use room temperature water when wetting the moss. This option can be less stressful for snakes who don't enjoy swimming.
The snake wants to get that retained shed off too. Leave the snake in the container with the moss for several hours. S/he will likely wriggle into the moss and hang out there until the shed is moistened. If you're lucky, the snake will wiggle around enough to get the stuck shed off all by itself. If not, you can try the damp towel routine again.
Other options include getting hold of the retained shed carefully with tweezers or fingers and letting the snake crawl off, thus pulling the shed loose. TYou can also try using masking tape, sort of the way it can be used to get lint off clothing. This won't work if the shed has started tightening up too much.
Be persistent. If not removed, a retained shed like this can result in a lost tail tip.