Route 109 Description

Pretty much, there's water, water and more water. There are small islands, and trainers, and deep water to Dive in (with proper equipment), where you can find treasures. Either way, nothing truly changes except for the weather, and trainers.

Your first time traveling is a bit rushed, as Mr.Briney runs his ship like a madman. so you really miss a lot of things. I can tell you though, that when the conditions are right... it's heard that certain Pokémon, such as Krabby, Kingler and Cloyster surf the waves. It appears that they've come from somewhere far away, but they are quite powerful. I've heard that it's been a while since they've shown up, and they only do so to worthy trainers, who battle from the backs of their noble Pokémon. Mr. Briney speeds you to the beach, so you can't really see the lay of the land from land, but there are some trainers, and a nice soda shop on the beach nearby. There's also some things hidden in the sand as well, so check things out before you go on to Slateport City.

But pretty much, if it swims, and is not a legendary or starter Pokémon, you'll find it in the ocean. just some things are far, far rarer than others...