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
; 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.