As above its no big deal because you wont have obstacles or sight impairments out on the water. Just set your battery warning to 30% to ensure you can get back.
Go to an open area like a sportsfield and practice. Gesture mode is a bit of a gimick. Play with it when you've got used to flying properly. There is lots to learn.
I've only had my Spark a week. Very much a beginner. I've already managed to destroy a prop by not setting the return to home altitude high enough to clear the trees! Live and learn.