try changing:
$WADbSearch1->addComparison("dt_Price","" . ($_GET['S_dt_Price'] -5000) . "","AND",">=",1); $WADbSearch1->addComparison("dt_Price","" . ($_GET['S_dt_Price'] +5000) . "","AND","<=",1);
$WADbSearch1->addComparison("dt_loa","" . ($_GET['S_dt_loa'] -10) . "","AND",">=",1); $WADbSearch1->addComparison("dt_loa","" . ($_GET['S_dt_loa'] +10) . "","AND","<=",1);
$WADbSearch1->addComparison("dt_berths","" . ($_GET['S_dt_berths'] -2) . "","AND",">=",1); $WADbSearch1->addComparison("dt_berths","" . ($_GET['S_dt_berths'] +2) . "","AND","<=",1);
to:
$WADbSearch1->addComparison("dt_Price","" . isset($_GET['S_dt_Price'])?($_GET['S_dt_Price'] -5000):$_GET['S_dt_Price'] . "","AND",">=",1); $WADbSearch1->addComparison("dt_Price","" . isset($_GET['S_dt_Price'])?($_GET['S_dt_Price'] +5000):$_GET['S_dt_Price'] . "","AND","<=",1);
$WADbSearch1->addComparison("dt_loa","" . isset($_GET['S_dt_loa'])?($_GET['S_dt_loa'] -10):$_GET['S_dt_loa'] . "","AND",">=",1); $WADbSearch1->addComparison("dt_loa","" . isset($_GET['S_dt_loa'])?($_GET['S_dt_loa'] +10):$_GET['S_dt_loa'] . "","AND","<=",1);
$WADbSearch1->addComparison("dt_berths","" . isset($_GET['S_dt_berths'])?($_GET['S_dt_berths'] -2):$_GET['S_dt_berths'] . "","AND",">=",1); $WADbSearch1->addComparison("dt_berths","" . isset($_GET['S_dt_berths'])?($_GET['S_dt_berths'] +2):$_GET['S_dt_berths'] . "","AND","<=",1);