My one recommendation is if you plan to do boat dives, especially while on tropical vacations, spend the extra $250 to get your Advanced certification.
Even if you don't ever plan to do deep, wreck diving, at night in a current, your experience will go a lot smoother than if you only have an Open Water certification. I've found that you just get less hassled and more respect...
However, if you just want a relaxing dive don't get your Rescue or Master diver certification (and if you do, make sure you also carry a regular card). That one guy who always shows up without a buddy? Guess what - he's your buddy now. Someone forgot to put their reg in before jumping in and got a lung full of water? Yep, you're up.
I'm not saying it's bad to have these - far from it - it's just that I've seen too many lazy charters think that you're slave labor.