You meet many people cruising, and saying goodbye is really only saying see you later, because sooner or later you will run into them again. It sounds strange, but even if your plans are to only gunkhole around the Chesapeake for the next decade, and the other's plan is to sail around the world - you will end up seeing them again. I can't explain it other than its a small world.
For example, we had an AC unit go bad, so we drove it to the manufacturer to repair. Walking in the door with us, with an AC unit in their hands too, was friends we had made 7yrs ago in Panama, but hadn't seen since.
Many, many of the friends you make will quickly recede to the background of your life. It is nice to run into them again, but not crucial. Often, occasional emails or other contact keeps them in your life. The good friends you make, then part from, you will keep in regular contact. This will be natural. You may even visit them occasionally (and vice versa) throughout the rest of your life no matter how yours and theirs proceed.
For example, we made some good friends and parted ways over 6 yrs ago. Farewell (for now) | East of the Equator | M&M
In the meantime, we have visited them several times while they worked in California, and are heading out next week to Turkey to spend a few weeks with them on their new boat there. During this time, we have kept in contact by usual means of email and social media.
So to me, the goodbye thing is just a natural way. It can be sad, but it is rational.