When I did my water line passing into my house through a concrete wall I left a square knockout in the form work. After stripping the forms and running the pipe through we filled the void with concrete. After it was filled with concrete the concrete wall was sprayed with damp proofing. After it was damp proofed I installed some rigid insulation and dimple board. Once the dimple board was installed we blue skinned the dimple board to the pipe where it went through the dimple board.
In your case, I'd probably dig a pit where it enters at least 12" lower than the entry point. Take some landscape fabric and make a wall around the pit. Blueskin your water service to the foundation wall. Fill the pit with gravel.
If it rains the ground water will have to come up at least the 12" for it to want to go into the garage. If it isn't too much work then dig the pit down to your perimeter drain where the water will be taken away ( assumes you have perimeter drain ).
Instead of silicone maybe try an epoxy putty.
This one is NSF and suitable for exterior use.
http://www.polymericsystems.com/technical-data/pdf/RepairitQuik.pdf