I always coach swimmers to look down towards the bottom of the pool as it helps maintain good posture and spinal alignment. The body works best in straight lines, like any machine. As soon as you start lifting your head, you either put pressure on your neck, by arching it, or your hips, causing your legs to sink as your body tries to keep that straight line.
But looking down doesn’t mean submerging your head underwater. It’s more about elongating your neck. This, allied with bracing your core, will keep you higher in the water and make breathing easier as you’ll only have to turn your
head to the side.
When it comes to open water, I think swimmers should keep things as simple, and as close to ‘normal’, as possible. Obviously you need to look up to sight, but in between sightings I’d go back to looking downwards. This will also reduce the risk of your wetsuit chafing on your neck.