On most cruises scientists work watches, i.e. fixed periods of the day of e.g. 6 hour duration. But this depends on what activities are going on at the time, and of course personal enthusiasm!!
On the G.O.Sars cruise this summer, many scientists worked much more than intended because they always wanted to be around when gears came onboard and new discoveries could be made. They then spent the day (and night) working up samples, monitoring videos from ROVs, and doing post-processing of data from various instruments.
Meal-times are fixed. Breakfast at 08:00, lunch at 12:00, and dinner at 18:00. An the mess was open to late-comers during the night! A favorite meal during the night this summer was sardines with a lot of Tabasco sauce.