Coal tar epoxies exhibit high water resistance characteristics and are good for water treatment applications. They're also often used for the exterior of pipes that are to be buried. The downside is that they contain carcinogens. One very general way to group epoxy systems is by generic crosslinker. Three of the most common are amines, amidoamines, and polyamides.
Polyamides tend to be lower on the scale of sensitization potential and higher on flexibility.
That is, they aren't as hazardous to applicators and they bridge cracks somewhat better than others. Fortunately, however, you can brush and roll them, which is not necessarily the case below. They also are more likely to cause problems if you get them on your skin. The mixing ratio is easy and the coatings are less susceptible to blushing, but they stink like heck, are hard to brush and roll, and they're difficult to overcoat once cured. Amidoamines fall between polyamides and amines, and can be a little trickier in application.