So far, so good. You might have problems if the two radios end up on the same channel (1, 6, or 11).
Well, no. The 2611CB3Plus requires that you type in the MAC address of the access point to which you're connecting, the SSID, and the channel number. Unlike most client adapters, it has no provision for scanning for available access points. It wasn't really made for roaming and was primarily intended for point to point links, where the other end is known and does not change.
For high power, I don't know. I'll see what Google finds.
Have you looked into commercial providers?
My ancient DLink DWL-900AP+ has a "scan for access points" feature. However, it requires that you pre-select the radio channel. That's kinda a problem if you don't know what it might be. It's also not high power.
You could just setup an ordinary wireless client on one of your desktops and install Netstumbler to search for access points. Scribble down the numbers (MAC, SSID, channel) and plug them into your existing 2611CB3Plus. Not the most graceful method, but functional.
Don't use high gain omni antennas on a boat. The -3dB vertical radiation angle is rather small. For example, a 12dBi omni antenna has a vertical beam width of only about 5 degrees. Your vessel needs only to roll about 2.5degrees from vertical and the signal is totally gone. Similar gain antennas in the form of yagi, panel, or dish antennas will have much larger beam widths, which can accommodate more rolling. Methinks a sealed panel antenna is the most common solution for boating.