if the ability won't matter to you then no, it isn't rare. You can fish for Feebas in Route 1, and with a high encounter chance too, if you fish on a water ripple.
It's hard to get, but not as rare as previous gens. I found one in the Dream World, didn't take it home because I met another pokemon that I wanted more, and I already had one in-game. I got lucky, I got in within 10 minutes of regular fishing.
3DS Friend Code: 3351-4057-4675 (Vicki)
2DS Friend Code: 0361-7983-6340 (Breanna)
Friend Safari X (3DS): Vicki ~ Rock withOnix, Boldore & Shuckle Friend Safari Y (2DS): Breanna ~ Ice withDelibird, Bergmite & ???