The Count, even if he takes the form of a bat, cannot cross the running water of his own volition, and so he cannot leave the ship. As he dare not change to man’s form without suspicion, which he evidently wishes to avoid, he must remain in the box.
– Bram Stoker
Dracula, Chapter 25. If the vampire hunters board Dracula’s ship by daylight, the Count will be trapped in his box, Jonathan Harker writes. So the group’s opportunity will be to get on the boat between sunrise and sunset, Van Helsing suggests.