Wet basements such as yours are often caused by conditions outside of and even on top of the house itself, so do some thorough investigation to locate other possible causes of the leaks. Clogged or insufficient gutters can be culprits, as can downspouts that send water straight toward the home's foundation (instead, downspouts should discharge at least four to six feet from the foundation). The angle of the soil around the foundation's perimeter can also be an issue, so work on adjusting its grade so that it slopes away from your home on a downward angle of six inches over the first four feet from the foundation wall. Finally, ensure that such landscaping touches as brick or wood edging, excessive topsoil or overgrown bushes and trees aren't holding water close to your home.
Going back to the legalities and warranty impacts of this situation, hire a lawyer immediately and put the builder on notice, as he is ultimately responsible for your wet basement woes. Also inform the warranty company of the situation and pending proceedings, as notification of the builder does not guarantee notification of the warranty company. Fast action is your best defense in this situation, both in terms of your home's structural integrity and protection against further liabilities caused by an unscrupulous builder.
Regarding the county telling you the home was built under a water table, I doubt it. Unless that advice came from the County's civil engineer, I wouldn't believe it. Homes are never built below water tables although that is generally the #1 reason give for floods by the uninformed!