Hello mate first off don't even think off failing if you want to be a diver then you'll become a diver, but yes if you do fail you get two options leave or branch transfer. I know a lad that's passed his aptitude and he gave me a little tip get fit as you can hint, hint.
Watch the video on this page of the RN website. Add being woken up in the middle of the night or early in the morning to plan evolutions, prepare sets and perform physically arduous tasks in and out of the water and you'll get the idea. Your mental acuity and determination, physical application, attention to detail, approach to team-working, concentration, temperament, ability to cope under pressure, regard for safe practice, strength and stamina will be assessed 24/7 but the time will just fly by because you'll be enjoying yourself so much. (Either that or you won't be able to think about anything else apart from what you're meant to be doing. ).
Just expect the unexpected, dig out, take one day at a time and you'll be fine. The RAN selection week shown in the first episode of the Navy Divers series on YouTube is very similar in practice and ethos but you are less likely to encounter a shark.