Place the egg yolk, mustard, lemon juice and salt in a large bowl and whisk together.
Mix the canola and olive oils together in a measuring cup. While constantly whisking the egg mixture in the bowl, slowly pour in the oil, drop by drop, until an emulsion forms.
Continue to whisk as you very slowly pour the oil into the bowl in a thin stream. If the mixture gets too thick, add the water to thin it out. Once about half of the oil is mixed in, you can pour the rest of the oil in a little faster.
When all of the oil is in, you should have a smooth mixture with a consistency slightly looser than store-bought mayonnaise.
At this point, taste the mayonnaise and season it to taste with salt and more lemon juice if desired. Stir in the chives. Store any extra mayonnaise in an airtight container in the refrigerator for no more than a few days.