The Iselin family archive, Basel
Skimming and scanning
You have a look at the material under Paragraph § 103. Apart from a file containing personal documents and three portraits on canvas depicting Isaak Iselin and his wife, there is a green leather bag containing Documente betr. die Reise von Isaac Iselin (Roulet) von New York, per «Maryland» nach China und zurück, 1805/1808. – und andere. This seems most promising.
Skimming the material, you see that at the beginning of the file, the archivist has collected descriptions of Isaak Iselin's life, both published and unpublished. They are followed by diaries from Iselin's time in New York and other materials. Overwhelmed by the richness of the bequest, you decide that you have to concentrate on your primary interest: the documents concerning the voyage of the Maryland and its sojourn in Hawaiʻi.