页面效果：

源代码：

<div id="water-wrapper"></div>
<div>
    <div>111111111111111111111111111111111111111111111111111111111111111111111111111111111111</div>
    <div>111111111111111111111111111111111111111111111111111111111111111111111111111111111111</div>
    <div>111111111111111111111111111111111111111111111111111111111111111111111111111111111111</div>
    <div>111111111111111111111111111111111111111111111111111111111111111111111111111111111111</div>
    <div>111111111111111111111111111111111111111111111111111111111111111111111111111111111111</div>
</div>

jquery版本

function water() {
    const watermarkText = '水印文字123'; // 水印文字内容
    const wrap = document.querySelector('#water-wrapper');
    const html = document.querySelector('html');
    const body = document.querySelector('body');
    html.style.margin = "0"
    html.style.padding = "0"
    body.style.margin = "0"
    body.style.padding = "0"
    wrap.style.width = "100vw"
    wrap.style.height = "100vh"
    wrap.style.display = "flex"
    wrap.style.position = "absolute"
    wrap.style.zIndex = "999999"
    wrap.style.flexDirection = "row"
    wrap.style.justifyContent = "space-around"
    wrap.style.flexWrap = "wrap"
    wrap.style.pointerEvents = "none"
    wrap.style.fontSize = "16px"
    wrap.style.color = "rgba(184, 184, 184, 0.6)"
    wrap.style.overflow = "hidden"

    const wrapWidth = wrap.offsetWidth; // 容器的宽度
    const wrapHeight = wrap.offsetHeight; // 容器的高度

    const tempSpan = document.createElement('span');
    tempSpan.innerText = watermarkText;
    tempSpan.style.visibility = 'hidden';
    document.body.appendChild(tempSpan);
    const watermarkWidth = tempSpan.getBoundingClientRect().width + 160; // 水印文本的实际宽度
    const watermarkHeight = tempSpan.getBoundingClientRect().height + 120; // 水印文本的实际高度
    document.body.removeChild(tempSpan);

    const numCols = Math.floor(wrapWidth / watermarkWidth); // 计算每行可容纳的方格数量
    const numRows = Math.floor(wrapHeight / watermarkHeight); // 计算可容纳的行数

    for (let i = 0; i < numRows; i++) {
        for (let j = 0; j < numCols; j++) {
            const watermark = document.createElement('div');
            watermark.classList.add('watermark');
            watermark.innerText = watermarkText;
            watermark.style.padding = "60px 80px"
            watermark.style.transform = "rotate(-45deg)"
            watermark.style.transformOrigin = "center center"
            wrap.appendChild(watermark);
        }
    }
}
water()