The reason is obvious.

The days are long and monotonous.

There is nowhere to go, nothing to see but the ocean, nothing to do but read, talk or promenade.

Seclusion in one's stuffy cabin is out of the question, the public sitting rooms are noisy and impossible, only a steamer chair on deck is comfortable and once there snugly wrapped up in a rug it is surprising how quickly another chair makes its appearance alongside and how welcome one is apt to make the intruder.
Thus events combined with the weather conspired to bring Shirley and Jefferson more closely together.

The sea had been rough ever since they sailed, keeping Mrs.Blake confined to her stateroom almost continuously.