I wrote a review trashing this game last week, but have given it some more time, and have come to like it a bit. It still needs a good deal of work though.
You can in fact play for free indefinitely, even working up to the largest ship of the fleet if youre decent at it. Paying for "pirate coins" will help you, but the cost is very high considering what you actually are getting. You can spend $1-5 real dollars worth of "coins" for a ship, which may get blown away in seconds, or if the game glitches, you lose it outright. If they were a better value, I might buy some. I have not as of yet due to bugs, crashing, and them being a generally poor value. If you insist on the biggest ship, you could realistically blow $20 in 10 minutes time.
Also, bigger isnt always better. It seems the big ships cannons are a bit less destructive on account of there being more of them.
The ad banners are very intrusive, popping up over the cannon buttons the instant you are sunk, and very often stealing an unintentional click, whisking you away to the app store. The game often crashes at this point, and you lose any progress you made. There is no way at all to pause or save progress either. Progress & gold coins are only good per session in a room, and are wiped anytime you leave or the game crashes.
The steering and aiming are not 100% reliable either, which is a frequent source of frustration.
All in all, I could see this game becoming a favorite if the developers put a bit more thought and care into it.
For the record, I am using a 3GS running 4.3.5.