\documentclass{article}
\usepackage{tikz}
\usepackage{calc}
\usepackage[letterpaper,top=0cm,bottom=0cm,left=0cm,right=0cm,marginparwidth=1.75cm]{geometry}
\begin{document}

\def\watermark{Exclusivement destiné à la location}

\newlength{\watermarklength}
\setlength{\watermarklength}{\widthof{\watermark}}

\def\step{\the\watermarklength}

\def\startX{-20}
\def\stepX{-16}
% \def\stepX{\pgfmathparse{\step+\startX}\pgfmathresult$}
\def\startY{-30}
\def\stepY{-26}
% \def\stepY{\pgfmathparse{\step+\startY}\pgfmathresult$}
% \def\stepY{8}

\begin{tikzpicture}[remember picture,overlay]

    \foreach \x in {\startX,\stepX,...,20} {
        \foreach \y in {\startY,\stepY,...,20} {
            \node[opacity=0.3, scale=1, rotate=45] at (\x, \y) {\watermark};
        }
    }
\end{tikzpicture}
\end{document}