My ducks always come back home from the lake. But they were raised here and know where dinner is. Also, I don't feed them during the day so they know if they want the good food, they have to come up in the evening for dinner and go in their safe pen.
Ours is a small lake, no boats except an occasional paddle boat, but ducks are pretty cautious anyway and are unlikely to get in the path of a boat, I would think.
Still, I would not expect them to know what to do if it's a vacation home and they haven't grown up there. If you're taking them on vacation, I recommend you take a portable pen of some sort or build them something at the vacation home to contain them. After a week or so, you could probably let them out and expect them to come back, but it takes them time to recognize their new home. Also, ducks are very territorial (as in, attached to their territory--not aggressive though), nervous creatures and they would probably prefer to stay home from vacation anyway.
Oh, and you probably already know this, but just in case--baby ducks shouldn't be swimming without supervision anyway, as they can get chilled, or not be able to get out, or drown. They don't have the waterproof coat yet, so they're much more vulnerable than an adult. Mother duck would make sure they came out of the water when they needed to, so you must take the place of mother duck when they hatch.
Enjoy your babies!