I agree that he was mostly running from Neptune Army, but he was not only running from the weak soldiers, but also from Fukaboshi and his brothers as I agree that he can't take three of them at the same time. Still I believe that Van der Dekken should be capable of worning out Ryuboshi as the fight will be as it seems on land, then I believe Ryuboshi's mobility would be limited, while Van der Dekken as a Fishman would have a physical advantage on land as they sem to be several times physically stronger than normal humans. And his attacks will be quite effective as if he sends tons of sharp things on a good speed, then I doubt Ryuboshi will be able to block or deflect all of them at the same moment from different directions.
The only flow of Dekken's ability is that he needs to touch his opponent to perform his ability, but since he has an upper hand in mobility, then I think he can manage to do this. We should also take in account that he was quite capable to keep his counciousness after being slashed by Hodi with an axe and we know that Hodi was damn powerful at that moment.
Thus I believe Dekken can win a fight, especially since it seems to be hold on land.
