One Piece is an anime stuffed to the brim with intense fight scenes and incredibly powerful characters. Many of these characters utilize Devil Fruit abilities, which they gain from eating an extremely rare type of fruit. This gives them interesting and useful powers but comes at the cost of losing the ability to swim in a world that is mostly ocean.
It's not always the best idea to set sail when a quick dip in the water means death. While Devil Fruits are a quick way to become stronger, they certainly aren't the only way. There are plenty of people in the One Piece universe who have attained strength and combat skills through other means.
10 Silvers Rayleigh
9 Vinsmoke Sanji
8 Roronoa Zoro
While Zoro is capable of using Observation Haki, he tends to throw himself into the use of Armament Haki. He is one of the strongest swordsmen in the series and has even gone up against a Navy Admiral. There hasn't been a rematch between him and Dracule Mihawk yet, but after training with him for two years he's bound to have closed the gap in strength and ability.
7 Dracule Mihawk
6 Jinbe
5 Monkey D. Garp
During the Marineford arc, he even said that Sengoku would need to keep him from killing the Admiral Akainu. Defeating an Admiral is no small feat, but coldly stating that killing them is a genuine possibility is quite the admission of power. There is little doubt that he would be capable of this feat, and he's already aged past his prime. There would be no reason for him to consume a Devil Fruit.
4 Ashura Doji
3 Kozuki Oden
2 "Red Haired" Shanks
1 Gol D Roger
