Whats the best way to patch a 1.5" hole in a poured foundation wall?
If the water is running through the conduit foam or duct seal would serve you better than cement. Duct seal has the advantage of not hardening, which should minimize the bill and foul language if you need an electrician to pull a new wire down the road. Then again, I'm not an electrician so there may be a better mousetrap.

To answer your original question "hydraulic cement" generally refers to products with a very short dry time that can be used in wet conditions. (Or under water) In addition they generally expand while they cure, as opposed to Portland cement which shrinks slightly. That's the big selling point for foundation work because it means there's no way for ground moisture to seep around the patch. 

The rationale for slight dampness is twofold. Masonry absorbs water, which complicates repairs. If you don't dampen the existing work it can draw water out of the new work and weakens it along the bond. In addition, products based on Portland cement requires ample water to cure but cure very weak if there's too much water. Generally that involves lightly spritzing the surface periodically during the cure so that it remains damp without standing water. Both are less of an issue with hydraulic cement simply because you wouldn't be paying the premium for it unless you were already working in a damp area.