After seeing “47 Meters Down,” I have a checklist of things to remember for my next Mexican underwater adventure.

First, do not accept a submersion in a shark cage from a shady character who keeps cracking “jokes” about how the sharks can hear you breathing.

Second, make sure somebody ties a bungee cord or something around the shark cage before you get inside, because it would be good to have back-up in case the chain holding the cage breaks. I’ve seen the movie, and it can happen.

Third, find out what a meter is. I am an American, so I have no idea, but 47 meters sounds like a long way down.

On reflection, scratch that last one. I don’t want to know how far down 47 meters is. Sometimes ignorance is bliss.

These helpful lessons may be the best thing to come out of “47 Meters Down,” a dumb movie that would be fairly suspenseful if only the characters could keep their mouths shut. (Yes, we can hear them talking underwater, thanks to the radio system in their scuba gear.)

The adventurers are two sisters (Mandy Moore and Claire Holt), who trustingly enter the shark cage owned by a crusty captain (Matthew Modine). When the winch mechanism breaks and the cage goes hurtling to the ocean floor, the sisters must battle fear, dwindling oxygen tanks, and of course the fiendishly large Great Whites circling them.

It’s easy to spot some of the predecessors here, such as “Open Water” (2003) and “The Shallows” (2016), in which stranded swimmers fight sharks. I suspect the example of “Gravity” is also lingering in the background, with water substituting for space.

Director Johannes Roberts blows some of the shock moments — at times you can’t see clearly what’s going on — but otherwise pushes the action forward. He wisely keeps us underwater, with the sisters, so we’re not sure what’s happening on the surface.

The dialogue is terrible. Moore and Holt are likable enough actresses, but they sound like Romy and Michelle on a deep-diving expedition. Shouting “That is like, the biggest shark!” is not a way to get the audience on your side, however accurate the assessment may be.

“47 Meters Down” (1 1/2 stars)

Two sisters (Mandy Moore, Claire Holt) vacation in Mexico and include a shark-cage dive in their fun. The film follows in the footsteps of “The Shallows” when the cage goes hurtling to the ocean floor and the women must fend off some very large Great White sharks. Despite some shock moments, a pretty dumb movie.

Rating: PG-13, for violence

Showing: Alderwood Mall, Everett Stadium, Galaxy Monroe, Marysville, Meridian, Oak Tree, Pacific Place, Woodinville, Cascade Mall