Well i went to boot camp almost 10 years ago to the day and back then it was nothing more than sleep deprivation and guys screaming at you 24/7. Now I hear the recruits get 8 hours of sleep, which is BS but well who am I to ruin you good luck. The Navy has changed a whole lot in 10 years and I'll keep my opinions to myself on that. Just be able to run 3 miles and do your sit ups and Push ups, also do pull ups because some of those rates you mentioned deploy and train with Marines so you will be doing Marine Corps PT.
As far as jobs go, If you aint an FMF corpsman you aint shit in the Navy. If you have the balls and are willing to work hard the Marines love HM's to death.
MA's are heavily over manned just like the Corpsman Rate is so dont expect to rise through the ranks at light speed. MA's can go to Mobile Security Det's and do some cool Anti Terrorism Force protection stuff.
CB's think they are Marines but do some cool stuff, forward deploy to some regions and do good things.
I'm aircrew and dont have much contact with those 3 rates but the military will give you what you put into it, keep that in mind. Good luck its an adventure.