With chartered flights, chances are you're departing either late morning or late in the evening. If you have a morning flight home, you have nothing to worry about. Just check out and wait for the bus to pick you up. However, if you have an evening flight, let's say 9:55 pm, the bus is going to pick you up around 6:00 pm. Sure, you can still enjoy everything the resort has to offer until you board the bus but do you really want to step on the plane smelling of sweat and sun?
You have two solutions: #1 - enjoy your day on the beach or pool and ask for a courtesy room to shower or #2 - ask for late check out. I would go with #2. There is no harm in asking. You will probably have to pay somewhere between $40-$60 for a 4 pm checkout but believe me, it's worth it. On your departure date, wake up early and check with the front desk. If they have availability, they will re-program your card and then you can enjoy your day and be all refreshed for your return home. If you choose to go with #1, no problem. Check out at the designated check out time, store your luggage with the bell boy (remember to lock your luggage and keep your sunscreen & valuables with you), check with the front desk about a courtesy room and see if you can reserve a time to use it and then head off to the beach.
Personally, we like morning flights and we aim to get those flights. We have in the past had some evening flights and we have done everything from storing our luggage and freshening up in our friend's room (they were leaving a week later) to actaully sitting in the lobby all afternoon after checking out to getting late check out. I highly suggest getting late check out, it just makes life so much easier!