Route 17 is an exclusively aquatic route, so expect to do a lot of surfing.
The Fisherman in the shallow water near the entrance has a Feebas L64, a Finneon L64, and a Lumineon L64. There's a hidden Blue Shard a little to the west of him, as well as another Blue Shard sitting out in the open on another patch of shallow water.
Head southwest from the second Blue Shard to find a Swimmer♂. He uses a Tentacool L65 and a Huntail L65. There are three water currents near him, although the top-most one is flowing toward you, so you won't be able to take it. The bottom one will take you on over to Route 18, but since it's a one-way ticket, you might want to check out the middle current first.
Taking the middle current will lead you over towards a Swimmer♀. She's got a Clamperl L65 and a Shellder L65. Head on over to the east of her to find another patch of shallow water, except this one with a Fisherman and a Dive Ball on it. The Fisherman uses a Krabby L64, a Qwilfish L64, and a Kingler L64.
Now that you've done that, head back to the stream and take the southern-most current to reach Route 18!