Click on Variables in the Code tab, then click on Make a Variable.
Type in the name of your variable. You can choose whether you would like your variable to be available to all sprites, or to only this sprite. Press OK.
Once you have created the variable, it will be displayed on the Stage, or you can untick the variable in the Scripts tab to hide it.

You can also choose a look for your timer by changing how your new variable is displayed.

Now add code blocks to your Stage so that the timer counts up until the boat reaches the island.

Test out your game and see how quickly you can get the boat to the island!