Preseason and regular maintenance checks are key
Owners are encouraged to have their boats looked over at the beginning of every season by a qualified person. If you are mechanically inclined, it doesn’t hurt to do a regular inspection yourself, to make sure bilge’s are clean, free of debris, hoses are without rot, checking carefully to ensure both gas and water hoses (especially through-hull hoses) are in good shape. Special attention should be paid to ensure stern drives, and the common accordion "bellows" for the stern-drive and shifter still have integrity. Turning the outdrive hard over each way and inspecting the bellows can sometime do this. Again, more with inboards and stern drives, make sure your blowers are working, and that all hoses are connected to allow the venting of gas fumes to be done properly. The structural integrity of the hull can be tougher and often required special equipment to measure moisture and the like. When in doubt, have an expert inspect your boat.
Expert Marine Surveyor, Joe Trudeau from McLarens Canada, International Loss Adjusters & Surveyors, points out that there may be a false sense of security and confidence in more modern materials like fiberglass. It seems too many of us simply slide the old fiberglass boat into the waters each spring, start it up and go.
The reality is that many of the boats have foam and/or wood core transoms, stringers and engine mounting points that with some age are in fact rotting badly. In some cases, depending on construction methods, even fiberglass hulls have picked up enough moisture to be weakened to the point of failing.

Related Downloads:
Boat Right, Be Polite fact sheet from Lake of Bays Association
Safety on the Water - from Ontario Ministry of Transport