Offhand, I can't think of any weatherproof powered wifi speakers.
However, if you are willing to move the speakers back and forth from inside to outside, multiple Sonos Play5's could work, as long as they don't get splashed. They can be grouped to play the same content stream.
Can you run speaker wire, perhaps some direct burial type? If so, you could locate a Sonos Amp in a spot that is convenient to run wire from, and use 2 pair of outdoor speakers.
Because the Sonos Amp is small and cool-running (Class D), you might even be able to create a weather-proof enclosure for it and place it in the most convenient location to run speaker wire from.
5.1 and 2.0 ch: Outlaw 975/Emotiva DC-1/Rotel RB-1582 MKII/Rotel RB-1552/Audiosource Amp 3/Polk LS90, CS400i, FX500i/Outlaw X-12, LFM-1/JVD DLA-HD250/Da-Lite 100" HCCV/Sony ES BDP/Sonos Connect. DC-1/RB-1582 MKII/Sonos Connect also feed Polk 7C in garage and Dayton IO655 on patio. 2.1 ch: Denon AVR-2807/Klipsch Forte I or NHT SB2/JBL SUB 550P x 2. 2.0 ch: Rotel RX-1052/Emotiva DC-1/Klipsch RF-7 III/Sony ES BDP/LG 65" LED. 2.0 ch: Klipsch Powergate/NHT SB3. Kitchen: Sonos Play5.