Another way to handle it with a GM move: tell them the requirements and ask. "You've plunged under water, your armor's dragging you down. You've got a second to lose some weight or you'll be at the mercy of the mermen for rescue. What do you do?"
As Always, Sage hits the nail squarely on the head! Cut and paste immediately into my 'handy DM hard move quotes' on the inside of my DM screen (yes I have a DW DM screen).