Be certain that Tiger doesn't eat any of those green apples just before boarding or while he is on the boat.

Green apples give cats tummy aches, causing them to forget how to swim and therefore to sink when they fall off of boats.

If you do feed him green apples while onboard, then keep a long dipnet handy, so that you can scoop him out the moment he falls into the water and starts to sink0000op[

(Everything that follows the k at the end of the final sentence above was written not by me but by a certain black cat, who, while sitting atop my monitor observing, had suddenly become extremely concerned with the situation and so stepped off the monitor onto the keyboard to express himself. Upon reading his entry, it is clear to me that he has analyzed the situation much better than have I and his solution is far superior to mine. Therefore, I suggest you ignore my advice and follow his, for he is the Black Cat and knows better.)