I thought I would report back in case anyone else is researching these subjects in future.

The Canal Transit. The tour was actually run by Bay and Canal Tours. I booked through Grayline partly because I had heard of them and partly because their booking site seemed fairly secure. They picked me up at the hotel though they did arrive early - a scenario I was not expecting. I now think that there is just the one tour and everyone is selling space on the same boat.

Taxis - I got a taxi from the airport which cost $30. It was all very organised and I was accosted by a man in a green "uniform" shirt and ushered into the first taxi in line. I got the hotel (Doubletree) to organise a car back to the airport because I am a wimp. It cost $35 and I had the option of paying the driver in cash or paying the hotel. I was happy with that. The man was there waiting at the appointed time and it was a nice, clean car.

I wish I had made more use of taxis as the road layout was incomprehensible!