I would use water plug cement. This fast setting cement is made to plug cracks and hole even with there is water running. You mix a small amount with water to make a stiff dough. Then push the dough into the hole or crack. Cracks should be chiseled out a bit. I believe the directions say to make a / \ notch, but I've found that is a bit hard to do and just opening the crack a bit will work. Mix only a small amount at a time and work fast as the setup time is very short and once is starts to harden, you can not add water so you may need to use several small batches rather than just one larger one. If I remember correctly, "Waterplug" is a brand name, but a decent hardware store should be able to point you to their brand of product.