Anything that you have to do or that would save your life must be done, for example if you have to eat than you must, you must break your fast and than make it up by fasting on another day during the year. the same thing goes here, if you must jump into the water, then that is  what you do and you can make up the qadha on another day in the year. as the principle goes that the most important thing here is life and therefore you should break your fast by jumping into the water. Secondly most of our scholars have the opinion that sinking into water entirely breaks your fast, however Syed Sistani does not.