Yeah Pirates is the one.
Many people (granted, they tend to be the fanatical players who only play Man o War) believe that Man o War is the best game GW has ever produced.
I don't play it... mostly due ot a lack of players, but more because I'm much more interested in modelling and being creative, but I agree - it was the best GW has made.
Unfortunately one of the main reasons it was so good was its undoing - you only needed a few miniatures, which meant people didn't need to keep buying more and more like they do with their other games.
I believe fantasy naval wargaming is a niche that really needs to be filled.
I am also willing to bet if someone were to produce one and it was a success GW and Privateer Press will jump on the bandwagon - there is already mild talk of a Warmachine naval game.
I myself have written some draft rules and can easily make ships... Problem is I'm hopeless when working alone - I get distracted easily. So I'm trying to muster up some supporters to lend a hand in whatever way they can, and maybe I can get something made.