Can you take your own boat from Vancouver to Seattle?
I don't know much about any of this, so any information on type of boat required, if there are specific routes you need to take, and how to travel by boat internationally (if it can be done/how you would go about doing so) is a great help.
Sure, there is no problem in doing that. You can take any route you like. After you enter US waters, you should endeavour to stop at the nearest port that has US entry facilities. There, you will talk to a US officer who will admit you to the United States (or not, as the case may be, no idea what might happen if you're not admitted at that point).
The US CBP has a web page on this topic titled What procedures must a small boat follow when entering the United States? which should provide all the information you need.
Greg Hewgill's answer is excellent in general but in specifics, I'd like to point out this advice from the Point Roberts Yacht Club:
Point Roberts is an excellent point of entry into US waters from Canada. You may clear customs by stopping at the Customs dock located on starboard as you enter the marina and phoning US Customs for clearance. There is a public phone on the gas dock and one on the main building by the Chandlery.
