Take a glass of fresh lemonade or sparkling water and toss it in the grape. It is a little heavier than water and sink to the bottom. But it will sit there and then the gas bubbles, like tiny balloons. Soon they become so numerous that grape pop up.
But on the surface of the bubbles burst, gas and fly away. Heavy with grape again sink to the bottom. Here it is again covered with gas bubbles and pops up again. This will continue a few times until the water "out of steam". According to this principle and pops up a real boat rises. And the fish have a swim bladder. When she had to dive, the muscles shrink, squeezing the bubble. Its volume is reduced, the fish goes down. A must rise - the muscles relax, spread bubble. It increases, and the fish floats.