"Best day of your life? l figured you should have a decent seat." - Flynn Rider, 'Tangled'
How did Rapunzel and her guide manage to find themselves in such a perfect position to look at the lanterns? Why were there no boats anywhere nearby, despite there being quite a few elsewhere? Wouldn't someone else want to get that kind of view? The cry was "To the boats!", but we see no boats other than the one our heroes are on.
I think this image largely explains it. The tiny rowboat is in a cloud of lanterns that have floated this direction, and are grazing the water's surface before moving on upward into the sky. Imagine if there were lots of boats, sailboats included, on this patch of water - it would be extremely risky for the lanterns (easy for them to crash and splash), and possibly also risky for the boats themselves (flammables near flames? No thanks). Flynn has taken his date to the quietest place around by the simple method of violating the kingdom's safety rule: No boats downwind of the island!
It makes sense. The weather report would tell them which way the wind's most likely to be blowing that evening. Some of the lanterns will rise straight into the air; others will climb for a bit, then settle down, and finally make their rise toward the sky, once the air inside them warms a bit more. (Some will be duds, of course, and won't get into the air. They'll catch the water and sink.) Anyone upwind or crosswind of the island will get a great view of the lanterns flying off into the distance, without any risks; downwind, all you need is a small exclusion zone, and everyone's safe. When the royal lantern comes down almost to the water, Rapunzel helps it on its way, but if she (and the boat) hadn't been there, it would have had enough room to catch the air on its own.
And isn't it perfectly appropriate for the thief ("lovable rogue", I mean, of course) to break the rules and get himself into the perfect, if risky, spot?