From the introduction:
Flotsam and Jetsam is [an organized play campaign] presented as four separate and loosely linked scenarios…
Each scenario is, essentially, standalone, and does not require the same players to take part, although returning players will, of course, enjoy a deeper level of fun by the fact they are engaging with a wider story. A set of pre-generated investigators is provided with the first scenario, and these may be reused in the follow-up scenarios; however, there is nothing to stop Keepers creating their own set of investigators, or for players to bring their own investigators to the game. Ultimately, the decision is yours as the Keeper of the campaign.
Each scenario offers one or more sessions of play. Keepers are advised to restructure, cut, expand, and do what they will, to meet the timeframes of their particular situation.