No one knows for certain what happened, but it's a fair bet that it was not an accident.
没有人知道究竟发生了什么,但几乎可以肯定这并非意外。
After it had passed, they swam on as quickly as they could because they knew that the boat would soon return.
快艇过去之后,他们以最快的速度向前游去,因为他们知道快艇马上就要转回来。