Welcome to Wonderland. That is the place that has its own rules, but nobody can tell you exactly what they are. Just ask anyone who moors a Canadian boat in Point Roberts Marina, or, as you do, keeps a US boat in a Canadian marina.
As a US Resident (I presume) none of the well publicised rules for returning Canadian residents apply to you. Every time you cross the border into Canada, you are entering as a visitor. CBSA will ask you what the purpose of your visit is. You will most likely be on vacation, so will answer appropriately. They will then ask if you have any alcohol, tobacco or firearms, (or not, if there is a long lineup they are more likely to wave you through). You know enough to leave all of your AT&F at home, so will move to the next probable question, "Do you have anything to declare". That question is really for a returning resident, not for a visitor, but if, in the back of your SUV they see boxes of stuff from a marine store, they may wonder out loud why you have such a load. In that event, I can't tell you what the correct answer is, as I have been unable to figure it out. When I kept my own boat at Point Roberts for a couple of years, I never got a meaningful answer from the CBSA or US Border Services guys, so I concluded that either they don't really know, or they just don't want to share. You might take the view that you are not bringing anything in for resale, but only for your own use on your own boat, and will be returning it all to the US when the boat leaves, so, really, there is nothing to declare. Or, you may take the cautious approach, declare everything and pay duty and taxes on it, just to avoid any uncertainty, that might lead to loss of your Nexus card or later difficulties at the border. If you take the latter approach, you will pay duty (usually 6.1%) on anything not covered by the NAFTA, and GST&PST (12% total) on everything.
Usually, telling the CBSA guys or gals what you have, so long as it isn't a huge amount of stuff, will get you waved through.
The same applies, in reverse, to Canadian visitors to the US.
Don't know Stan's background, but I do know that a boat seizure will only happen if the boat is your mode of crossing the border. You are asking about driving through, so the car could be subject to seizure if, when they suspect you of smuggling, or worse, you get searched and that kg of coke becomes an issue. I have never heard of anyone losing their car for failing to declare a depth sounder.
