If you rented a standard week (Saturday to Saturday) or any other 7-day period, you need to be back at the base marina the evening before your rental ends: on Friday for a Saturday-to-Saturday week. You can still sleep on board on your last night. For other rental periods, ask the charter company; the return procedure can differ.
Why the day before?
Check-out takes time. A diver checks the boat below the waterline, the charter company inspects its general condition, settles your bill and asks for your feedback. Doing this the evening before means everything is sorted before you leave.
It also gives the charter company time to repair any damage before the next guests arrive, which matters when they need to order and fit spare parts.
Check-in and check-out times for a weekly rental: What time is check-in and check-out for a weekly rental?
