Iowvis,
25 yards underwater should be easily attainable; but not be swim conditioning. As some others have stated, it involves technique and relaxation. My stroke is a modified breast stroke, whereby I make a a full, body-length pull with my arms starting in front, ending at my sides (thigh), and gliding; then I recover my arms to extend them in front during my frog kick, and glide some more. I repeat this stroke until I have swam the 25 yards (which is the width of my community pool. DO NOT EVER TRY UNDERWATER SWIMMING FOR DISTANCE, as that is a prescription for shallow water blackout. But 25 yards should be easily and comfortably attained.
SeaRat