In swimming, there is nothing solid to push against; therefore, though strength does matter in swimming, it is much more important to apply pressure to the water in the correct way. Because water is a fluid, it moves around you rather than resolutely resisting you in the way that a solid does. Similarly, speed in swimming comes not necessarily from moving your body parts faster but from using your movements to put pressure on the water in an effective manner. You can work awfully hard in the water yet go nowhere.
As with fins, great care should be taken when using paddles because the increased pressure on the water creates extra stress on the joints.
Breath holding causes tension, which can lead a swimmer to sink or struggle with balance; exhaling slowly, on the other hand, aids relaxation and is also a necessary skill for performing the competitive strokes. To control your breathing, rather than exhaling forcefully, breathe out through your nose while humming.