There are a few reasons why Mr. Narracott, the boatman who brought all the guests to the island, does not return at this point. The primary reason is the weather. Even though the storm is gone, its severity continues to affect the sea. Recall what Lombard says while the remaining group of people are eating breakfast:

The weather’s cleared all right, but the sea hasn’t gone down yet. Terrific swell on! They won’t be able to get a boat near the island before tomorrow.

This line tells the reader that Mr. Narracott has not returned with a boat because the waves are still choppy and stormy. Waves like that are dangerous to ride a boat in because it could make it difficult to see and thus navigate clearly.

However, there was another reason why Mr. Narracott has yet to return. The person who is behind this whole deadly game hired a man to do some of the work for him and instructed this man to tell Mr. Narracott not to return until a particular date and time. Mr. Narracott was also told not to return even if he saw signals asking for help or indicating an emergency. In the end, Mr. Narracott ignores those instructions and arrives early anyway after hearing that some Boy Scouts saw SOS signals. Inspector Maine reflects on Narracott’s decision to do this and thinks he did it because of how strangely normal the guests were to be stuck in such an odd location.