While signaling doesn't seem as important for small boats leaving slips in a marina, it can be very important for larger vessels that can't start and stop on a sand dollar like little outboard boats. Even so, it's not a bad habit to adopt. Prior to backing, you should sound one long (two seconds will do) and three short (a second or less) blasts on your horn, then proceed. The signal doesn't require agreement or response from surrounding vessels.
Approaching a Bridge
Most drawbridges have marine VHF radio communications. Often, the expedient way to get a bridge to open is to call the tender on channel 13 or 9 (whichever is used locally). However, in the rare case where the bridge tender has no radio or has taken a nap, the appropriate horn signal consists of one long blast followed by a single short blast. Standard procedure dictates the bridge tender warn boats with five short blasts when the spans start to close. If you happen to be under the bridge when you hear those horns, respond immediately with five or more short blasts so the tender can hold open the spans until you're clear.
In a Fog
The rules of the road clearly state that if you are in fog and you hear another vessel's signal, you must slow dramatically or stop until you determine there is no risk of collision. If you hear a horn in the fog, know that it's a vessel underway. The specific combination of signals will tell you the kind of vessel. If you hear a bell or gong, that's a vessel that is stopped for some reason - probably anchored or aground.
A powerboat underway in fog must sound one long blast every two minutes or less.
If that boater is stopped (but can maneuver), he must sound two long blasts about two seconds apart at intervals of two minutes or less.
A vessel engaged in fishing (lines, nets or trawling) must sound one long and two short blasts, again at intervals no longer than two minutes.
Interestingly, this is the same signal required of a sailboat under sail. Go figure.
Anchored boats shorter than 39.4 feet LOA must make some noise (preferably with a bell) every two minutes or less. Vessels larger than 39.4 but shorter than 328 feet must rapidly ring a loud bell for about five seconds at intervals of one minute or less.
Like auto insurance, this information is something you might never need. However, suddenly needing it even once will make your effort to know these rules very worthwhile.