According to the nearest weather station, Cherry Point South Dock Met,
station CPMW1, sustained southerly winds that afternoon went as high as 19 knots, with gusts to 21 knots. At the same time, a south-setting tidal current off Lummi Island was running around 1.5 knots in opposition to the wind. Having paddled that stretch of Rosario Strait myself many times, I can confirm that a 19-knot wind against a 1.5-knot current would, indeed, generate occasional wind waves in the three to five-foot range, although most of the waves would come in around half that size or less. Those wind, wave, and current conditions are easily enough to swamp or capsize a kayak. A sufficiently skilled kayaker could pull through unscathed without too much trouble, but I suspect most regular folks would find themselves in "survival mode."
I too have paddled that section of water many times, and I have often stood at Stony Beach, or Sunset Beach on the North end of Lummi Island and looked at conditions when the wind was blowing against the tide. There are very often significant rips that develop in Rosario Straight where it intersects with Georgia Straight between Matia and Lummi Islands. I sometimes head out from Sunset Beach to play on the edge of said rips in windy conditions. Just speculating, but it is likely that these folk were returning from Matia or Sucia Island. That is a big open water crossing.
The other factor that is interesting is the two double kayaks with a dog and a child in the centre hatches. The Coast Guard said this in response to a query on their Facebook page:
Despite their training in kayak retrieval, the challenge of occasional 3-5 foot wind waves proved formidable. The kayaks remained intact but were swamped by water from the surging waves, not from any breaches.
I have paddled a double kayak frequently, usually with less experienced paddlers, including in this very same area. I have also often paddled my double with my two young grandsons, one in the front cockpit, one in the centre; but only in calm water.
Some things I am convinced of about after paddling a double for many years, even a very sea worthy one:
1; A kayak of over 20 feet long and more than 30 inches wide is much harder to handle in rough water than a single. I have played around and practiced surfing a double, and it takes two very skilled paddlers to keep it from yawing and broaching in surf. Lots of fun at places like Pachena Bay, but lots of rolling and swimming as well.
2; Same with broaching in big waves. A double has vastly more surface area on the sides, this means when it is hit by a wave, it requires a skillful and coordinated effort from both paddlers to keep a big wave from rolling it, this is done by edging and bracing correctly. I have more than a few times had to shout at a less skilled paddler to lean and to brace to keep the double upright. That often does not have much effect and I have been left to try to keep the boat upright by myself. Preferably in a double in rough water, both paddlers have the skill to react without consultation or instruction. A less skilled paddler in a double panicking or reacting badly to a big wave can easily cause a capsize.
3; If the centre hatches were open due to the child and the dog, dumping waves were likely filling them.
The kayaks remained intact but were swamped by water from the surging waves, not from any breaches.
That would lower the boats in the water and make them heavier and more unwieldy, even for two skilled paddlers. If the centre hatches on the boats were open and taking on water, it would be impossible to pump or bail them out.
4; It is a fallacy that a double kayak is more stable and less likely to capsize in rough conditions. My personal opinion is that this particular myth is one that sometimes gets people to launch into conditions they might not attempt if everyone was on the beach with single kayaks. It takes the same skill level to paddle a double in rough conditions, as it does to paddle a single. That means both paddlers must have good skills and also preferably practice in paddling a double.
A double feels safe and stable in calm water, not so much in rough water. Many folk who paddle double kayaks are totally rudder dependant and feel that other than steering, they don't have much to worry about in a double. The other skills necessary to paddle confidently in rough conditions, leaning, edge control, and bracing are not often understood, considered or learned.
I know of only one couple who could actually roll a double, they practiced in a pool to learn to do it. It required bomb proof rolling skills and enough practice so it became truly instinctive, right down to the timing of the set up. They used to demo it a the various kayak symposiums once held in the Pacific Northwest many years ago. Muscle memory, timing and reflexes honed by practice.
I am very glad that these folk were rescued and that the Coast Guard was there for them. Hopefully they learned from this experience. The place where this rescue occurred is big open water that can be deceptively placid looking, but if the wind picks up or the tide changes, it can get very technical in a hurry.
For the rest of us, if you paddle double kayaks, practice with them in rough conditions in a place where it is safe to do so. Practicing in small surf is fun and you will learn a great deal quickly.
Double kayaks are great seaworthy kraft, but a craft is only as good as the skills and experience of it's paddlers. My two bits worth;
Cheers, Rick