Since this is Pretty Little Liars, we don't believe in coincidences. So here are three theories about what Rosewood High's creepiest couple was really doing at the lake that night:
1. Rescuing Lucas, accidentally: One theory is that they happened to be in the right place at the right time, saw what went down, and rescued Lucas. If that's true, the question becomes: Why didn't they tell the truth? It's possible Lucas asked them not to, or maybe they're keeping the secret for their own, less honorable reasons.
2. Rescuing Lucas, because they're working together: Alternately, it's possible that they rescued Lucas because they're all secretly working together — maybe even as "A." Mona and Lucas both have plenty of reasons to hate the Little Liars, and we've never trusted Noel after he blackmailedEzra (Ian Harding). Maybe Lucas did have something more nefarious than a confession of feelings planned, and his partners in crime were waiting on the shore as back-up.
3. They were spying on Lucas and Hanna: Or maybe they were spying on Lucas and Hanna's little rendezvous. We could see Mona and Noel following the pair out to the boat for any number of reasons, from being "A" to simply wanting some juicy gossip.
Or maybe they really were just taking a romantic dip.