Ground Water Seeping in Through Foundation Joints

We live in a townhouse development. The hurricane drainage results in groundwater seeping into our home in the basement where the two homes meet between our unit and a neighbors. We believe this is a foundation deterioration. It is a fact our units are built on an underground stream that overflows during extreme conditions. Each rain, more water inflow. Recommendations? Thank you.

The Money Pit Answer

In my experience, homes are rarely built on streams even though homeowners often feel passionately that they are.  The fact that your unit leaks consistent with rainfall further emphasizes by belief that this leak caused by drainage - and can be corrected with proper drainage and grading.  
Before you do anything, though: In a townhouse form of ownership, you might not be responsible for correcting this problem.  Depending on how the public offering is written, townhouse owners often own from inside of sheetrock to inside of sheetrock.  If this is the case for you, it could be the responsibility of your association to investigate the problem and make repairs.