Some people I role play with want me to run a pirate themed game and I was wondering if anyone knew where I could find some info on pirates in the old world, mainly around Lustria and Cathay. 
Pirates, Arr!
I don't know for sure, but I think this game will have some pirate info. I don't think it will be around the locations you asked about though ):
Although this is written for the 2nd ed it should have some good source material, also do a google search for any warhammer saratosa (pirate island in the araby sea) stuff should prove useful.
There are a huge range of fan materials etc out there that should not be too hard to convert to 3rd ed.
Alp
Edit should have added this address http://www.strike-to-stun.net/download/The_Thousand_Islas.pdf
Here's where I'd start:
Novel
- Fell Cargo
Fan Supplements for Tilea:
WFB Tilea
WFRP Tilea (by bateman) - the thousand isles
Fan supplement for Cathay
WFB Cathay
Look around for information about the Island of Sartosa. While it isn't near Lustria, it is the "centre" of pirate culture in the Old world, being one big pirate haven off the coast of Tilea.
My group ran a 2nd Ed pirate campaign based there, although I don't recall the sourcebooks the GM used.
2nd edition sourcebook The WFRP Companion features an article on Sartosa, "city of pirates". Also, it has an article called Life and Death on the Reik that has some info on pirates in the Empire.
I'll second the recommendation of the Dread Pirate game for sea warfare...that is, if you've got the extra cash to spend. Otherwise I would definitely look into some of the older Warhammer FRP 1st Edition & 2nd Edition supplements mentioned above. In addition I would recommend Green Ronins "Pirate's Guide to Freeport". It's system neutral and would make a great addition to the various Old World islands out there. I've considered plopping it down between Lustria and the Old World in the middle of the ocean, for PCs interested in sailing to Lustria it would make a great pitt stop.
Thank you guys some of that stuff is going to be really useful, now I've just got to write up and adventure