Kat and KH, if you're reading look away; this has major spoilers for our game.
I'm currently DMing a homebrew 5e campaign. My players are unraveling the Mystery of the Missing Cargo, which is basically a convoluted, fleshed out game of Knight Knave Normal. There are three ships in the harbor, cargo is going missing and they're all varying levels of eager to help the PCs figure out what's going on.
As a red herring, the knaves are making it out that mermaids are stealing the cargo by seeding dropped scales, "mermaid sightings" and more. The knaves will eventually turn out to be stealing cargo for the local thieves guild and pointing fingers elsewhere to keep the heat off their backs, but I'd like to disguise that fact for a while longer.
My players are currently having a great time following the mermaid trail, and are planning on camping out on one of the ships (the knights, as it turns out) to see if they can spot anything.
My idea was for the knaves to have a high level spell caster aboard (it's already been established that they have some pretty high level crew) who is helping maintain the illusion. I'd like some kind of effect that will serenely blank out the night's activities, without alerting the PCs that magic has been preformed upon them. There are actual mermaids (there's a whole sub plot involving them that the PCs are just starting to crack into) and they've already had a run in with Charm, which I've slightly modified to act as a "siren song" effect.
What I Need
My players are both level 4's, and one's a half-elf who has
advantage on saving throws against being Charmed, and magic can’t put you to sleep.
Neither specializes in magic beyond basic cantrips. (One's a ranger/cleric, the other has chosen to go full tank and knows no magic).
I need some way for them to be "distracted" while on watch, allowing the knaves to slip aboard and make off with cargo. Ideally, this would be subtle and not alert the PCs that anything was out of the ordinary. It's also acceptable to have your solution point fingers at the mermaids being the culprits.
Requiring a save is fine as long as it's highly unlikely they'll make it; a little bit of danger keeps it fun ;)
I'm open to existing spells, methods of disguise, illusions... anything that won't alert the PCs. Bonus points if you can show me a loophole/problem that the PCs could pick up on if they were looking for it. I'll accept answers straight from the book, or tested homebrew.