Whenever the boat is stored in the water, it's in a pitched battle to keep out all the water surrounding it, and that battle gets considerably more difficult when ice might damage a thru-hull or bilge pump, when the electricity might go out causing batteries to go flat, and when snow buildup in the cockpit might submerge above-waterline fittings. If you are going to leave your boat in the water, make sure you pay careful attention to the following areas as well as to your battery.
Thru-Hulls And Other Below-Waterline Openings
If the boat must be left in the water, all thru-hulls, except those for cockpit drains, have to be protected by closing all seacocks and gate valves. If your boat has thru-hulls below the waterline that can't be closed, it should be stored ashore for the winter. Raising and refurbishing a boat that sinks is a daunting job that can keep the boat in the repair yard for many weeks over the spring and summer. And all thru-hulls, especially the ones for the cockpit drains, must be double-clamped with stainless steel hose clamps at each end. When water freezes, it expands and will lift a poorly secured hose off a fitting.
After the seacock or gate valve has been closed, remove the hose so that it drains and then use an absorbent cloth or turkey baster to remove any residual water, which can freeze and crack the body. Reinstall the hose immediately and secure the two hose clamps.
Thru-hulls above the waterline are not required to have seacocks and most don't. That doesn't mean that these thru-hulls aren't vulnerable. Ordinary plastic thru-hulls crack and deteriorate in sunlight, but that won't sink the boat until the weight of ice and snow in the cockpit forces the thru-hull below the water. Plastic thru-hulls near the waterline are especially vulnerable and should be replaced with bronze or Marelon.
Removable knotmeter impellers and depthsounder transducers, if any, should be removed and replaced with locking dummy plugs. And if your stuffing box (where the propeller shaft exits the hull) is dripping, adjust the nut until the dripping stops. Check for leaking in the rudder stuffing box(es) if your boat has one.
Exhaust Ports
It is advisable to plug exhaust ports when a boat is stored in the water because if snow piles up on the stern, exhaust ports get pushed below the surface. Plugging exhaust ports will also prevent unwanted guests from finding their way aboard. There have been several claims involving muskrats chewing their way through parts of the exhaust system, sinking the boat. Don't forget to remove plugs in the spring!
Bilge Pumps
If the bilge pump runs, it means your boat is taking on water — or, to put it another way, your boat is sinking (albeit slowly). There's no acceptable amount of leaking. Make sure the bilge is free of any debris or oil that might clog the pump or interfere with the switch, and that the bilge pump is wired directly to the batteries (with a fuse) so that it will operate when all of the switches are turned off at the main panel. Also check the operation of bilge pumps. After you've cleaned the bilge, add enough nontoxic antifreeze to trigger the float switch. Bail or sponge the remainder out.
Docks And Docklines
Nylon lines stretch and absorb shock, which is good, but this stretching chafes the lines against chocks and other contact points. Chafe guards should always be used on lines when the boat is left unattended for long periods of time. Ready-made, polyester chafe protectors are available from marine chandleries or you can make your own out of heavy-duty polyester and Velcro.
A dockline is usually the culprit when a boat sinks after being caught under a dock. This occurs more frequently in the winter, probably because of the stronger winds and higher tides. Centering the boat in the slip and using long docklines and springlines led at shallow angles will help to keep the boat well away from the dock. If your boat is tied to pilings, consider using TideMinders or other systems designed to allow the line to slide easily up and down the piling without getting caught.
(Thanks to BoatUS)