Hi Dan,
Greetings from the south side of the pond.
I assume your crossing does not cross the international border, as that opens up a big can of worms.
September is right at the peak of salmon fishing season, so it may be difficult to hire fishing
charter boats. Also, the second weekend in September has been terrible
weather for at least the past four years running. You may want to think about late August.
One of your best sources for accompanying vessels would be a yacht club. They have boats (both club-owned and member's boats) that
work the races. On a non-race day you may be able to coerce them to
work your event.
If you can get
small boat owners interested they may do it for the
price of
fuel; if you need to
charter crewed boats figure $600-$800 each (US$, rates for the US shoreline, don't know about Canada)
You may also want to start the race earlier to avoid the mid-day breeze, which can be substantial.
Chuck