You don't usually need to buy much before you go that isn't just common sense. Good shoes, some sandals for the beach, and not much else.
Anything uniform-related, you actually have to buy on board. It's quite the racket, actually. Some companies even make you buy their brand of shoes (which is actually kind of it's own little racket on it's own).
Almost all crew members are going to be on a ship long enough to go somewhere in a third world country, where it's actually cheap to buy clothes...so I wouldn't bother paying big prices in the US, when you can buy something cheaper when you get on the ship.
If you need suits, or formal dresses, that's different...but as far as boat-clothes. Haha...forget it. You're in a hotel, or on a beach. There are Walmarts everywhere...and there are usually crew shuttles in every port anyways.
You can't get around buying uniforms onboard anymore...they gouge you. It comes right off your first pay, and they make you buy all their stuff. Sometimes you can get lucky and bring your own stuff and wear it, but you still have to buy what they sell you. I take khaki cargo pants/shorts, and a couple pairs of black pants...that i never wear anyways.
Hope that helps...don't feel the need to spend money if you don't have to. You'll be spending enough when you get on the ship...and you'll be kicking yourself for buying too much before you go.
I pretty much lived in my uniform, and hardly wore anything but. I was always working. If I wasn't...I had like one outfit that would last me days, because I'd only wear it for a couple of hours a day. Off the ship...it's no different than home...really.