Well, I located an ipx to SMA dongle locally, but can't find a SMA wifi antenna in retail stores. I reside in Asia and rf equipment is hard to come by due to government regs.
Having studied ham radio as a kid, I decided to search the web for DIY wifi antenna solutions and was delighted with several easy alternatives.
A. One can make a flat dipole out of copper, possibly using printed circuit board. This is a nice compact design for an interior use.
B. One can make an extremely directional Yagi antenna for outdoor use where one might even be a mile away from one's wifi modem. VERY INTERESTING!
The trade off is omni-diectional antennas are low gain; whereas, the high gain one are narrow beam diectional.
I have not provided DIY links, but search "DIY wifi dipole" or "DIY wifi yagi antenna".
So a Banana Pi might be adapted to be a long distance wifi computer to be used by your swimming pool.
I can modify the dongle to fit whatever I choose to build.