My first open water dive was at 70 feet off Delray beach, my wife and I had private lessons and gave us the luxury of each having a divemaster.
I agree 15 feet feels like 60 feels like 145', but things happen very quickly deep. Make sure you understand deco, and have a plan that you stick with. If you have the luxury, do a simulation with your computer down deep, review how the machine alerts you to deco and what it tells you to do, make sure you figure in additional stops even though the computer tells you where the deco stops are. I
I have been stopping at halfs recently, spending a minute until I hit 20 then the three minutes if possible.
Two final things, practice a skill a dive, flood mask, change second stages, reach around and touch your tank valve (going after lobster under a ledge, my valve bumped a few times and closed, needless to say I was able to reach around and turn it back on, however, now I practice it fairly often). Always make sure you know how much air your buddy has, try to predict prior to them answering you.
Every once in a while get in the pool and remove all your gear and put it on underwater, do it in reverse.
Good luck and be a little paranoid, it is safer.