JB Water Weld
This stuff is just unbelievable when it comes to repairing anything related to water. You can even repair something 'under' water with it!
Also it's incredibly cheap too.
Sets up so hard you can file it down if needed. No mixing 2 parts either. Just pull a piece off and roll it in your hand. Feels like clay and when the color changes your ready to patch. Ace Hardware carries it.
I did around my shower drain that started to crack.......4 years ago and still holding strong!!
http://www.jbweld.com/product/j-b-waterweld/