The Yachtmaster Offshore preparation course is a 4-day course focused on fine-tuning your existing Yachtmaster-level theoretical and practical skills, and practising various boat handling scenarios.
The prep course is followed by 1-2 days of examination with an external RYA examiner. Please refer here for more information on what the course does and doesn’t cover: https://www.bluewater-sailing.net/rya-yachtmaster-preparation-course-caribbean/
If you do not feel you are already at Yachtmaster Offshore standard, then you need to book tailored training beforehand to bring your skills up to that level.