link to Semiahmoo Marina
Here is a link to the Semiahmoo Marina web site.
http://semiahmoomarina.com/
Like previously mentioned, Point Roberts would be closer if you take the ferry from Sidney but if there isn't a slip available then obviously Blaine is the next stop.
Semiahmoo Marina would be a few more miles driving around Drayton Harbor but it is a really nice marina and there are a number of Canadian boats moored there. They do require proof of insurance to keep a boat there but then this is pretty much standard procedure throughout all the marinas now.
The sailing "outside" Blaine and Point Roberts tends to be very nice from what I've seen. No significant hazards to navigation.
When entering Drayton Harbor there is a starboard piling that acts as a buoy outside the entrance. Keep a bit of distance from it as it dries at low tide.