When one is working about the boat and dock, they could easily fall in. Make sure you have an exit strategy and not an "it wont happen to me" attitude. Extra caution should be taken in northern climates to assure your ability to get out of the water. Going in the drink with extra cloths on and cold temperatures could likely be fatal, if you can't muster the upper body strength to pull yourself out.
If your boat has a ladder on its swim platform, make sure it is down when your boat is tied to the dock. You may not be able to lower the ladder, if it is up when you are in the water. On my boat the dinghy rides on the swim platform, further restricting access to the platform.
A simple boarding ladder, available for about $40 from your local boat supply, is the answer. If you can't have it hanging on the end of your dock full time, just leave it on your swim platform for retrieval if needed. Secure it with some line to prevent it from being knocked off by visiting creatures.
Sometimes the simplistic fixes and some forethought are the best rescue tools we have at our disposal.
Boy Scout Motto "Be Prepared!"
Boaters Motto " It's not if - it's When"