Is there some type of cruise/ferry I can take from Nice, France to Barcelona, Spain in September?

There will be plenty of options once you get there. I have traveled extensively around Europe and the best thing to do, when you get there, is ask!
If you don’t feel comfortable doing that – have you tried checking with a travel agent?
Ferry/ Boat is a very popular form of transport for the Europeans – I have used the Jadrolinija service a few times (Croatian based ferry, but they do some international) Hope this helps…

Ahh yes, luckily I’m going with a French native, but he hasn’t been back in about 5 years, so we’ll see how much he knows!

Look, im sure you will be prefectly fine! Even when things go wrong, its usally a great experience! Hahaha either that or just a great story to tell ;)
Hope you have an absolute blast!

I can understand why you want to get out of nice. There are so many ferries leaving nice… You should consider separate trips so as to immerse yourself in one culture or the other… but if in france I would consider a trip to the music festival in san remo (Italy, if the timing is right) I have a house in Eze… but so far from there, have taken a private seaplane into Ibiza… (tho I love spain) you have a french boyfriend? OY! good luck to you! remember to breath.