Hop Hop Hop Underwater is a game developed by Ketchapp you can find both on App Store and Google Play.
In the game you control a character which can hop above and under the water.

When above the water, you must tap to jump. When underwater, you must tap to dive.
We can build a HTML5 prototype in a few lines using Phaser, this way:
Tap to hop if you’re above the water, or to dive if you are underwater.
In the completely commented source code, you can see it’s just a matter to switch player gravity at the right time, as well as inverting the thrust if we are underwater.
The concept is: jump force must be always against gravity direction, have a look at the source code:
let game; let gameOptions = { // water level, in % of screen height waterLevel: 50, // ball gravity ballGravity: 1600, // jump power ballPower: 800 } window.onload = function() { let gameConfig = { type: Phaser.AUTO, backgroundColor:0x888888, scale: { mode: Phaser.Scale.FIT, autoCenter: Phaser.Scale.CENTER_BOTH, parent: "thegame", width: 750, height: 1334 }, physics: { default: "arcade", arcade: { gravity: { y: 0 } } }, scene: playGame } game = new Phaser.Game(gameConfig); window.focus(); } class playGame extends Phaser.Scene{ constructor(){ super("PlayGame"); } preload(){ this.load.image("ball", "ball.png"); this.load.image("water", "water.png"); } create(){ // add water sprite this.water = this.add.sprite(0, game.config.height / 100 * gameOptions.waterLevel, "water"); // set registration point to top left pixel this.water.setOrigin(0, 0); // set display width to cover the entire game width this.water.displayWidth = game.config.width; // set display height to cover from water level to the bottom of the canvas this.water.displayHeight = game.config.height - this.water.y; // add ball sprite this.ball = this.physics.add.sprite(game.config.width / 2, game.config.height / 4, "ball"); // set ball ballGravity this.ball.body.gravity.y = gameOptions.ballGravity * (this.isUnderwater() ? -1 : 1) // listener for input, calls "jump" method this.input.on("pointerdown", this.jump, this); } // method to make ball jump jump(){ // set ball velocity to positive or negative jump speed according to ball position this.ball.body.velocity.y = gameOptions.ballPower * (this.isUnderwater() ? 1 : -1); } // method to check if the ball is underwater isUnderwater(){ // true if ball y position is higher than water level return this.ball.y > game.config.height / 100 * gameOptions.waterLevel } update(){ // determine next ball gravity let nextGravity = gameOptions.ballGravity * (this.isUnderwater() ? -1 : 1); // is next ball gravity different than current ball gravity? // this means we moved from land to water or from water to land if(nextGravity != this.ball.body.gravity.y){ // set ball ballGravity this.ball.body.gravity.y = nextGravity; // set ball velocity as if we just jumped this.ball.body.velocity.y = gameOptions.ballPower * (this.isUnderwater() ? 1 : -1); } } }
I am using Phaser for years, but I always get impressed by how easy is to build hyper casual games with it. Next time we’ll add some enemies, meanwhile download the source code.