After the credits, you get a message saying "Do you want to keep playing?". There's two endings, so i'm assuming if you hit no it takes you back to the checkpoint at the end to see the other.
I hit "Yes" and finished off everything i had left to do on the southern island, no problem. I could even load it up now, with everything done except some hunts, missions and treasures to find and still play.
TL;DR Yes, you can keep playing.
This is what I thought, but hitting no just takes you to the main menu, so unless youfor a back up save you have to beat the game again to see the other ending. Fine by me now I want to do hard