Waterside definitely has two bedrooms convertible to three. There aren't any listed at the moment, but they will be. There are usually a lot of vacancies over the summer as NYU students graduate and move out. It does not require a guarantor or a co-signer. You'll just need proof that you attend NYUCD. I used my acceptance letter as proof. They have a long history of working with NYU students and they know that we're good for the rent. If you're planning to move in early August, I would suggest starting to look at apartments in like.. June basically. That should be plenty of time.
Also, PM me if you decide to do Waterside. They give a $1000 referral if you put down a current tenant's name on the application, and it's customary to split that referral 50/50.