其实原理就是控制正弦波的幅度。

CSS:

html, body {
  height: 100%;
  text-align: center;
  background-color: #323436;
}

canvas#sineCanvas {
  width:500px;
  height:500px;
  overflow:hidden;
  background:#5b5d5e;
}

HTML:

<div id="canvas-container">
    <canvas id="sineCanvas"></canvas>
</div>

JS:

(function () {

var unit = 100,
    canvas, context, canvas2, context2,
    height, width, xAxis, yAxis,
    draw;
var percentMax = 0.6;//液体占容器的百分比
var percentMin = 0;
var maxHeight = 1;
if(percentMax>=1){
	maxHeight = 0.95;
}
var wavePower = 3;
//初始化
function init() {
	window.requestAnimFrame = (function(){
	  return  window.requestAnimationFrame       ||
			  window.webkitRequestAnimationFrame ||
			  window.mozRequestAnimationFrame    ||
			  function( callback ){
				window.setTimeout(callback, 1000/60);
			  };
	})();
    
    canvas = document.getElementById("sineCanvas");
    
    canvas.width = 1000; 
    canvas.height = 1000;
    
    context = canvas.getContext("2d");
    
    height = canvas.height;
    width = canvas.width;
    yAxis = 0;
    
    draw();
	
}
function draw() {
    
    // 清理canvas画板
    context.clearRect(0, 0, width, height);

    //描绘波浪
    drawWave(['#cea155','#e8c183'], 1, 1.5);
	
	if(percentMin<=percentMax){
    	percentMin = percentMin+0.002;
    	xAxis = Math.floor(height*(maxHeight-percentMin));
    }
    if(percentMin>0.80){
    	if(wavePower<=36*percentMax){
        	wavePower = wavePower+0.05
        }
    }
    
    // 更新时间然后重新绘制
    draw.seconds = draw.seconds + .009;
    draw.t = draw.seconds*Math.PI;
    window.requestAnimFrame(draw);
};
draw.seconds = 0;
draw.t = 0;

/**
* 描绘波浪
* drawWave([渐变左侧颜色，右侧颜色], 不透明度, 波浪的幅度)
*/
function drawWave(color, alpha, zoom) {
    context.globalAlpha = alpha;
	var linearGrad = context.createLinearGradient(0, 0, width, 0);
	linearGrad.addColorStop(0.0, color[0]);  
    linearGrad.addColorStop(1.0, color[1]); 
	context.fillStyle = linearGrad;
	
    context.beginPath(); //开始路径
    drawSine(draw.t / 0.5, zoom);
    context.lineTo(width + 10, height); //路径朝canvas右下
    context.lineTo(0, height); //路径朝canvas左下
    context.closePath() //封闭路径
    context.fill(); //涂色
}

/**
 * 画正弦
 * drawSine(时间, 波浪的浮动)
 */
function drawSine(t, zoom) {

    var x = t; //将时间转换为x轴
    var y = Math.sin(x)/zoom;
    context.moveTo(yAxis, unit*y+xAxis); //在开始位置放置路径
    
    // 绘制波浪、横向幅度 
    for (var i = yAxis; i <= width + 10; i += 10) {
        x = t+(-yAxis+i)/unit/zoom;
        y = Math.sin(x - 0)/wavePower;
        context.lineTo(i, unit*y+xAxis);
    }
}

init();
    
})();