How do you extract line items from invoices that are underneath watermark text

You can use the globalTextFilters that will remove text from document before it will be parsed.

For example, if your invoice has the watermark CUSTOMER COPY above the table text you can use the following expression in the template.

"options": {
        "ocrMode": "auto",
        "ocrLanguage": "eng",
        "globalTextFilters": [
            {
                "expression": "CUSTOMER COPY"
            }
        ]
    }