Set the size of your container and float your text using absolute positioning while transforming your text with rotate.
#watermark {
height: 450px;
width: 600px;
position: relative;
overflow: hidden;
}
#watermark img {
max-width: 100%;
}
#watermark p {
position: absolute;
top: 0;
left: 0;
color: #fff;
font-size: 18px;
pointer-events: none;
-webkit-transform: rotate(-45deg);
-moz-transform: rotate(-45deg);
}
<div id="watermark">
<img src="http://www.topchinatravel.com/pic/city/dalian/attraction/people-square-1.jpg">
<p>This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark. This is a watermark.</p>
</div>
Note: For repeating text, I would suggest using either JavaScript or jQuery.