To give an example to back up what SNS3Guppy posted.
Many years ago I operated a Westwind II to Hawaii from San Francisco. The Westwind had a marginal dry footprint for the crossing, there would be times we would have to wait for a day or two for favorable winds to make the crossing to assure a dry footprint. Then during recurrency school a pilot I knew related an incident that happened to him about an hour out of Guam heading for Wake Island to refuel on a flight to Hawaii. A little over an hour out of Guam the hydraulic up-lock line on the right main gear blew and the right main gear came down. With a main gear hanging down you are in very serious trouble on a long over-water flight. You cannot maintain speed, altitude and the fuel burn goes through the roof. If he had been another two hours into the trip he would have had to ditch, with the gear down. The odds of survival are very slim.
After hearing this I informed the owner that I would no longer operate the Westwind on Hawaii flights until the manufacture developed a fix for this problem, if in fact they ever would fix the problem. He informed me that he did not think that it could happen to us and I had problems with that I could quit.
So I quit.
Sure enough about six months later the same thing happened to his aircraft, fortunately on a flight over land. He sold the Westwind II and bought a Jetstar II.
Point of this story is, a good competent crew takes any and all possibilities under consideration, even the type of aircraft that will be operated on long over-water flights.
Just to assure some of the people here reading this, most all aircraft have mechanical gear up-lock on the landing gear. If the up side of the landing gear hydraulic system should have a leak or lose a hydraulic line the gear will remain up and locked. I know all Boeing aircraft do.