That Alfa sells for $35 on Amazon then all you need is a longer USB cord to get it out of the cabin. Yes, the USB cord length limits position but for $35 versus 250+, I'd try it first.
You aren't giving the resellers credit for the weatherproofing and technical support.
I experimented with an Alfa, a Bullet, and a Port Networks bridge before landing on my own solution. I've also installed some Bullets and some Wiries (Alfa) for clients.
On Auspicious I have a Bullet with 8 dB omni mounted above the radar on an aft radar pole. The Cat 5 Ethernet goes to a POE injector behind the electrical panel. A Linksys WRT-54GL is under a locker nearby. Everything runs off 12VDC on a "network" breaker on the panel.
Between laptops, iPhones, iPod Touch, and other odds and ends I've had up to eight devices sharing the boat WiFi network. At the moment it's just my boat computer, my lab laptop, iPhone, and the iPod Touch streaming Netflix to the TV (*grin*). The community wireless AP I'm hooked up to is about half a mile away.
I do still keep a standard Alfa in my boat bag for trips. It works okay but isn't anything like weather-proof and keeping the drivers working is a pain.