You don't need a water ramp to do a cork 7. If you can do it on a trampoline first and then try on snow, if you don't have access to a trampoline just try it on snow.
A larger jump will make it easier to a cork 7, pick a jump that you can comfortably straight 720 on. I learnt to do cork 7's on a small crappy snowdome jump that has ice for a landing just got to give it a go. Probably the most important thing with a Cork 7 is to be patient on the take off, if you spin to early like I did on my first go you will probably end up having a ski come off on the take off which isn't fun.