I'm about ready to pull the trigger on a mizzen mounted WIFI antenna and router because I can't easily get access from the marina where I keep my boat.
There are a number of options.
From an architectural point of view there is an external access device--a router--that connects to the shore side WiFi access point, a power-over-Ethernet (poe) injector that uses the Ethernet cable to the router for power to the router, and an internal router to disseminate Internet access inside the boat (although you can use wired Ethernet instead if you prefer).
USB-connected devices like the Alfa are simple and inexpensive but their performance is not nearly as good. Radio sensitivity is well below the standard set by alternatives. In addition the drivers required by USB are prone to failure which locks up your PC (Windows or Mac). It is interesting to note that The Wiries, based for years on the Alfa, has finally redesigned their product to use a Ubiquiti Bullet. I carry a USB Alfa in my computer bag for WiFi range extension on the road and in bars and cafes. I'm the guy sitting in the corner pounding on a laptop. *grin*
The Bullet has been the WiFi range extension gold standard for several years. The Bullet on my boat has been in operation for almost seven years. There are a number of sources for systems based on the Bullet. Note that many are relabeled and some have custom firmware intended to simplify the user interface. The latter point is not as wonderful as you might think as if the Bullet locks up (more likely with custom firmware than factory firmware) and you reset it you will have the factory firmware and a new learning curve. Frankly I think the factory user interface is pretty straightforward. For an internal WiFi router I happen to like the Cisco Linksys WRT-54GL. It is reliable, robust, and runs off 12VDC. I spoke with the engineering folks (not tech support) at Cisco last year after they released a bunch of new WiFi products and they continue to recommend the WRT-54GL for applications like ours. There are other options available.
A second option is the Microtek Groove, most famously available from IslandTimePC. Bob Stewart's customer service is first rate and he can sell you a complete kit including mounting brackets. The biggest difference between the Bullet and the Groove is that the Groove does not have a bridge mode which you will never use anyway.
The third option is the RedPort Halo. I have a Halo on my boat that has been in operation for about two years for side-by-side testing. The Halo performance is equivalent to the Bullet. The Halo is available with either of two matched internal WiFi routers (called an Optimizer) that have some unique capabilities that are attractive most notably a firewall that can be toggled on and off that keeps Windows, MacOS, iOS, and Android operating systems and applications from "phoning home" to download huge updates. When bandwidth is limited and/or paid for by the bit this is a HUGE deal.
To integrate cellular (3G/4G/LTE) Internet access you need an additional radio. That can be built into the WiFi box (as The Wirie does it) or a cellular radio connected to the internal WiFi router. Unless you will manually decide which route to take to the Internet (an approach with some merit) the internal WiFi router must be smart enough to manage access. There are two good solutions to this part of the problem: one of the range of Cradlepoint products (the solution of choice in the RV community) or the RedPort Optimizer Premiere (my recommendation). I like the Optimizer for automatic failover, the firewall, and the in-built support for just about every satellite system on the market. If you have or might add a satellite phone the Optimizer is a no-brainer.
For the actual cellular radio you still need a service provider. For cruising in the US either Verizon or AT&T are good choices. Both have hardware that connect to the internal router using USB. Verizon coverage is marginally better and AT&T speed is better. I do not recommend Sprint, T-Mobile, or any of the regional carriers unless your cruising grounds are very limited. If you leave the US (even just to the Bahamas, Canada, or Mexico) AT&T is your best choice with a quad-band or quint-band radio stick that is unlocked for local SIMs in your international cruising grounds.
For your installation the best locations for the external WiFi router/antenna are on a radar pole aft, on a backstay, on the pushpit, on a spreader, or a dorade guard. Mounting at the masthead is likely to distort the antenna pattern of your VHF radio and can reduce performance if you are located too close to the shore-side access point. A mizzen masthead is fine although it is better reserved for an AIS VHF antenna and mount your WiFi router/antenna somewhere else. The internal WiFi router should be mounted quite low and centrally in the boat. The Wirie internal WiFi router is in the external box which is why I still do not recommend it even though they have converted from the Alfa to the Bullet. You want your internal WiFi low and central to: provide good coverage throughout the boat, limit range to the boat, avoid radio co-channel interference between the internal and external WiFi routers, and avoid radio co-channel interference with the shore-side access points to which you are trying to connect. Be sure to secure your internal WiFi router.
Disclaimer: I sell this stuff. I can sell you a system based on the Bullet, the Groove, or the RedPort Halo. You will get a better price on the Groove from Bob Stewart at IslandTimePC; I can't compete with his volume on Groove. I can pre-configure your system to make it more plug and play. Within some limits I can customize mounting. There are other people who can do the same work but most marine technicians just open the boxes and plug things together. If you are in or passing through the middle Chesapeake Bay or are where I happen to be on a delivery or speaking engagement I also do owner-assisted installation. I can support DIY installs by phone, Facetime, nearly any IM service, and email.