Subject to any ideas they can get from phoning a friend on the network (Marcos does know a lady who can make rock walls??), my only thought is to have one of the three act as bait to distract the lead mermaid while the other two sneak inside as they're sleeping. Or two distract, one goes in. It depends on how that would go.

Marcos can make bright lights/signals or shoot off rocks and things with his laser hands, but if they are blind they wouldn't see the light probably, and exploding rocks could compromise the cave and wake the sleeping mermaids. This is what I'm working with...

I'll have more thoughts later I'm sure, just brainstorming!