By the way, I did HIIT today, 10 times 2 min sprints with 1 min heart rate recovery, add to that warming up and cooling down, I managed to collect 8.5 km
Definitely not. Either just below or just above 90 degrees, never at 90 degrees. That's way too much pressure on your knees. Also, I'd tend to go deeper as you get more hamstring and glute involvement that way. Almost all research these days says deeper is better, if you do it with good form.
I ran only once on the beach and I did that barefooted. It's tough but it builds up muscles in your feet that you cannot build up by any other means of running. I have never tried running on the beach with running shoes but even with shoes they say it's tough.
Do I recommend it? Of course I do but not as your only terrain. Variation is the key to successful running.
It is extreme fun and the best part of it is because it's really exhausting and sweaty but you are so close to the water that you can take a swim just after. I tell you, that run was one the finest I've done.