More vessels sink at the dock while unattended than sink while underway and manned.  A common reason for these sinkings occurs when cockpit drains get clogged and the vessel takes on water from heavy rains or waves slapping over a low transom.  The added weight of this water lowers the vessel in the water until a through hull fitting or cut-out transom is forced  under water.  Sea water then back siphons into the hull from the fitting or floods over the transom sinking the vessel. 

The average vessel's bilge pump system and battery capacity is not designed to deal with this amount of flooding, especially when unmanned.  Often the bilge pump(s) onboard are just sufficient to dewater a hull from normal water accumulation such as a dripping stuffing box or spray and rain water that finds its way into the hull.  The amount of flooding that occurs when a prop shaft falls out of a vessel, or from a lost sea cock, is substantial.  Flooding of this magnitude would require several high capacity bilge pumps (e.g. 3000 gph),

Your best defense against a dock side sinking is to check on your vessel often, and ensure that cockpit drains are kept clear of debris.  In addition it is important to check and maintain all through hull fittings.  Plastic through hull fittings are notorious for degrading from UV exposure and snapping off at the slightest pressure.  If your plastic fittings leave a chalky residue when wiped with a finger, replace them now!  And take the opportunity to upgrade to a bronze fitting.  Hose connected to above water through hull fittings should lead upwards if possible.  The higher the hose is lead above the waterline, the lower your vessel can be submerged without creating a back siphon.  Finally, if your vessel has a low transom (as found on many outboard powered vessels) be sure to dock it with the bow of the vessel pointing to any exposed stretch of water.  That way, storm waves will break on the bow rather than over the transom.