This question is often asked due to your country FCO advising against "all but essential travel to all islands off the eastern coast of Sabah". To be able to answer this question we need to understand how FCO advice works, know more about the current safety situation in Sabah and what security measures are in place to keep Turtle Island safe.
FCO advise is typically based on their assessment at the time of a security incident and not necessarily based on the current situation on the ground. As such travel advisories tend to err on the side of over-caution and their advice can take months if not years to be downgraded regardless of actual conditions on the ground.
In Sabah there have been a small number of abduction cases / attempts at kidnapping between 2000 – 2016 with the Abu Sayyaf group being the main perpetrators for most cases. To address this security threat the Eastern Sabah Security Command (ESSCom) was established a few years ago. ESSCom strives to reinforce security measures in the east coast of Sabah utilising military patrols, police and Maritime units. An enforced night time curfew on boats also helps by restricting undetected incursions. It is fair to say that since these measures have been taken, no security incidents have taken place so far.
Turtle Island is safe to travel to with the military, Pasukan Gerakan Am (PGA), and ESSCOM both stationed at Turtle Island helping make sure tourists are safe when visiting the islands. The Resort is typically fully booked most days throughout the year with no known security issues ever happening at Turtle Island - so many visitors to Sabah have decided the risk level of visiting Turtle Island is acceptable to them. Where it is impossible to give a 100% assurance of security, we believe that Sabah is 'very safe' and we feel comfortable in sending our guests to visit Turtle Island.
Ultimately it is a personal risk decision if you choose to go to Turtle Island. You need to decide if based on the known facts on the ground at the time if they represent a risk level to you that you're happy to take.