congratulations on such a sweet gig! SO to piggy back on what @dragonflyAS has mentioned, you will want to fly at an angle not parallel to the canals, a 30-60 range should work nicely. There are a couple of reasons for this; it will allow you to capture better the edges of the canal where parallel travel will loose some data there, this will also increase the amount of data points you get that are outside of your desired area (in this case the canal) this is important for the software to be able to create accurate tie points, and will help to increase the local accuracy (resolution) of the map. Make sure you are flying more than just the 75 feet width of the canals.
As to the height this is counter intuitive, you might think the closer i am the better detail i can capture in a photo, and for a single photo this may be true, however you have to remember that a key point of producing high quality and superb definition orthos is the ability of the software to find and coordinate tie points to stitch the images together. So you will find there is a balance here, the lower you fly the more detail in one picture, however since you are actually capturing a smaller area in the photo you are getting less data for the software to use, which will lower the accuracy. On the other hand if you fly higher you will be capturing more data in each photo for the software to be able to create more tie points. This will lead to an overall better map quality ( i would suggest running some test runs to find the best mix of detail and data for your needs.)
Also since you are talking a very large, if not just long, project make sure you are breaking the entire project into an organized system, (section A- map A1, etc) this will allow for easier organization and file management. and make sure your mapped sections have some overlay so they can be tied together.