In Ultra Sun, there are unfortunately no Pokemon that can learn both Yawn and Fake Out that can breed with the Blastoise line. However, in Generation 7, both the male and female breeding partners are able to pass down egg moves, which means you can breed a female Squirtle/Wartortle/Blastoise with one of the egg moves with a male Pokemon that knows the other desired egg move.
For Yawn, there are several Pokemon lines who can pass down Yawn to a bred Squirtle, being the Slowpoke line, Snorlax, the Wooper line, Kingdra, Relicanth, and the Bidoof line. The easiest to obtain out of this list would be Slowpoke, which can be found at Route 1 with the move Yawn already learnt. Catch a male Slowpoke with the move Yawn and breed it with a female of the Squirtle line. All the hatched Squirtle through this method should have the egg move Yawn. If so, keep breeding until you get a female Squirtle, which will take a while, as Squirtle have low chances of being female.
Now for Fake Out, there is only one Pokemon that can pass down the egg move Fake Out to the Squirtle line, being Lombre. Lombre learns the move at level 16, and is obtained through the Ultra Space Wilds, which you should make sure the Lombre you have is male. Though, as its level will be at 60 once caught, you'll need to head to the Move Relearner in Mount Lanakila within the Pokecenter at that location for the price of a Heart Scale. Once you taught the Lombre Fake Out, breed the male Lombre with the newly bred female Squirtle which has Yawn. Now after all that work, you should have hatched a Squirtle with your desired egg moves.
