The single most critical reason boats are flooded on open water has to do with transom height. Thirteen of 15 boats in the sample group that were swamped were outboard powered, with engine cut-outs that were often only inches above the waves. (Of the two remaining boats, one was an inboard with very low freeboard that took a wave over the bow and the other was a sailboat that was knocked down and sank when water entered a unsecured cockpit hatch.)
Motor wells are supposed to be the second line of defense when a wave comes over an outboard's transom but, in some cases, the well is too low, too shallow, and/ or not sealed adequately to the cockpit. Scuppers in the motor well and cockpit may also be slow to drain, especially if they re clogged. And whenever water lingers in the well or cockpit, the chances of another wave coming aboard increases. So too is the risk of being swamped.
Aside from transom height, the other contributing factor when a boat is swamped is typically weight distribution-- too many people at the stern together with scuba tanks, large coolers, bait wells, etc. that reduces buoyancy aft. In most cases, the boats were stopped or idling. The one exception was a boat that broached while entering a breaking inlet.
It should be noted that boats under 20' are required to have level flotation, so many of the boats in the study remained awash, although several were rolled over by the waves or by passengers rushing to one side of the boat.
Prevention: Especially on outboards with low cut outs, be conscious of weight distribution. Avoid storing scuba tanks, heavy coolers, etc. near the transom At slow speeds, keep the boat moving toward the waves. Don't anchor from the stern.
Most scuppers are slow to drain anyway, but when they're plugged up with leaves and other boat-gunk the water can linger in cockpits and motor wells a dangerously long time. Use a dockside hose with a power spray nozzle to flush out debris.
