I'm a Canadian living in Canada, and sailing on a lake that straddles the border.
An I-68 form, or a Nexus card will facilitate entry into the US by boat from Canada. Think of it as a restricted pre-approval for entry in uncontrolled areas. You are still required to "call in", but the process is much more streamlined.
An RABC (Remote Access Border Crossing) is the same idea for crossing into Canada from the US. The main difference being that if you aren't bringing anything into the country then you don't have to check in. BE ADVISED: If you get caught bringing something in without declaration they can get a might twitchy.
Overall, I've found the process of going from one side to the other by boat to be quite painless from a customs perspective, and quite liberating from a cruising perspective.
Edit: I should add that cost wise the Nexus card is cheaper than the I-68, but as of the end of this summer season it didn't work coming into Canada.
Last edited by PaulfromNWOnt; 10-20-2011 at 12:10 AM.