If you used the 4 tables from round 1 (taken from Whist 16) in order for the first four races, and then the next 4 races were taken from round 3 in reverse table order, then the 5th race would not contain any people from the 4th race and so the waiting would be minimized. I am not certain, but I think it should be possible to rearrange all the rounds like this, some the right way around, some in reverse until the whole schedule was played out.
But I am not sure that I have fully understood the question, surely 8 boats is enough?
Ian.