It was suicide, really. Anyone with the most basic level of common sense would agree. To leave the protection of the traveling ship while on these perilous waters was akin to self mutilation at best, and immediate and unquestionable death in any other circumstance.
But the two young lovers had, to a large extent, become numb to the sensation of danger. It wasn't because they expected Cliff to help them - he stayed true to his word and never interfered. At most, he would offer them an early warning should they be about to encounter an enemy far beyond their cultivation level.
The reason they had become numb was because they had, time and time again, overcome unbelievable odds through their advanced techniques and incredible teamwork. While sometimes they became a little conceited, it only lasted until the next time they encountered danger to remember that even their techniques and the tips on their teamwork came from Cliff, so they were never ungrateful to him.
