It's a difficult choice as all three are slightly different beasts, I have so far purchased Ports of call and Virtual Sailor and tried the Ship Sim demo
Virtual sailor is certainlly the biggest with the most addons and a large community. It really depends what you are looking for I view Virtual Sailor as being similar to MS Flight Sim there is a lot there but there is no driving force pushing the game along, you make your own fun, no missions or challenges. The cheapest game of the three Virtual Sailor is only $25. Note that it might be worth waiting until V7 is released so you don't have to pay again.
www.avsim.com/vs
"Ports of Call" is also open ended, but a Tycoon style game, the player's goal is making money and building the largest fleet. At is core, it is a ship management game with optional 3d or 2d simulation elements eg docking, navigating through icebergs rescuing survivors etc. The full version is about $49
www.portsofcall.de
Ship Simulation 2006 is I believe going to be mission based complete a mission unlock a ship, kind of thing which gives the player a goal to achieve. It's got 40 missions plus you can create your own, I think it's early days for this sim but it has potential. And it is a good buy at only $38 approx.
www.shipsim.com
Great idea on DW by the way.