You row to shore to take care some errands and tie up to a dinghy dock. The dinghy is going to be floating there unattended for some time and there can be a lot of people walking past it. What's to say one of them won't untie the dinghy and go on a joy ride? Or worse yet, simply untie it and set it adrift? The little bow line you have is no deterrent as it can be easily untied.
What can you do to help improve the chances that the dinghy will still be there when you get back?
Chain and lock the dinghy to a fixed object
Cable lock the oars and outboard motor
Lock the lifejacket or take it with you
Hide the dinghy
When you chain the dinghy to a fixed object, make sure that it is not possible to slip it off the fixed object. When you put a chain around the piling, make sure the locked chain can't be slipped over the top of the piling. I prefer to lock to the docks under structure. These tend to be large timbers that will keep the chain trapped in place. The only way to get the chain out of the structure is to unlock the chain and slip it out.
It is also important to cable lock the oars and the outboard motor. Outboards sell quickly and are easy to remove from the back of the boat. Passing a cable lock through the bracket of the outboard and then through a cleat or around a bench will help deter the theft of the motor. Also be sure to pass the cable through the fuel tank handle to keep it from wandering off.
Oars are difficult to lock up since they are long and have no holes to pass a cable through. For this reason, drill a small hole in the blade of the oar and pass the cable lock through them to keep the oars from walking away. I personally don't lock my oars at this time because no one is interested in oars, they all want to steal outboards.
The lifejacket is another easy to take item in a dinghy. You need to have one on board for the trip from yacht to dinghy dock, but what do you do when you leave the dinghy? You can either pass the cable lock through it as well as all other valuable items in the craft, or take it with you as you run your errands. I personally take it with me because an empty dinghy is less interesting than one that is full of items to take.
My favorite way to prevent dinghy disappearances is to hide the dinghy from view. I like to tie the dinghy up where it can't be seen from the street. This way, fewer people will notice it and hopefully a potential dinghy thief will not be enticed to visit the dinghy pier. If it is a very low tide (and I'm going to be running a very short errand) I will slide the dinghy under the pier between the pilings. When the dinghy is hidden properly, all that will be seen from the dock is the small line tied to a piling. If no one knows that the dinghy is there, no one will bother it.
The dangers of placing the dinghy under the pier is the rising tide could damage and sink it. If the tide comes up, the dinghy will be trapped under the dock and the rising tide can flood the boat. The other problem is the topsides of the dinghy can get bashed as the craft hits the underside of the pier and pilings.
A chain and cable lock are not a guarantee of dinghy safety, they are merely a deterrent. If someone wants to steal a dinghy, they are going to choose the easiest one in the bunch. The goal is to not be easy. If they have to cut a lock, cable, or chain to get the dinghy; they will probably go for the one that is merely tied up instead.