Is it possible to create a totally custom itinerary or can I only choose from ones that are suggested to me?
Everything is possible.
Suggested itineraries have been created out of many years of experience in organising yacht charters, with feedback from charterers too. They tend to maximise experiential opportunities whilst taking into account practical navigational matters — which may include, for example, shifting places overnight when guests are asleep so they can wake up in a different setting.
At the end of the day, every charter itinerary is custom made and can even be customised almost « on the spot » should you wish to.
It is advisable to take advantage of the knowledge your captain will have garnered. He will often be able to suggest an itinerary that will surpass than what you had in mind purely through his experience.