The Syrens Wifi system is just a packaged marine antenna, bridge/router. It is a good way for them to make a good deal of money for not offering a whole lot. You could do just as well or better by getting a PCMCIA WiFi card with an external antenna jack and mounting an antenna on the boat... like a 90˚ 9dB gain planar antenna... and aiming it manually. The 90˚ coverage is the minimum you'd want, as the boat will swing a little. You could go with an omnidirectional, but the gain would be lower. The 200 mw PCMCIA cards are the ones you want to get for this type of use.
The advent of the 802.11n specification, which is not yet finalized, will improve range quite a bit. The pre-specfication gear for it is already out there, but as it is fairly new, unlikely to be deployed in many areas yet. It uses multiple radios
, versus the single radio
in the 802.11a/b/g standard equipment, to increase throughput and range significantly. The 802.11 pre-n gear is backwardsly compatible with 802.11b/g gear, but with no real benefits.
You do need to be near someplace with a broadband internet connection that has a WiFi access point for either of the setups to work...neither will do much for you on the open ocean or in remote anchorages.
The other option, as PBz has mentioned is connecting via a cellular telephone network. However, outside of the US, most of the cellular data plans are based on the data sent, not a flat monthly fee... so they can really add up. I've used T-mobile's cellular network for my internet connection when I was in the process of moving, and it's not all that bad.... faster than a dial-up modem, but slower than my current fiber optic connection.
The only reason a steel boat would affect any of this is if the antenna was down below, inside the boat... and it would basically punt... steel eats WiFi signal really nicely.
BTW, I've run a WiFi network at distances over a mile using 18dB gain parabolic grid antennas... but that was with two terrestrial, non-floating endpoints. The record for WiFi is somewhere over 40 miles IIRC.