Personally, I believe that the boats should charge the appropriate rate for the charter to adequately compensate the crew. Than tips should be given individually for extraordinary service. As is, I just add 10% automatically when I book the trip. I consider it a part of the charter cost that the fleet is to cheap to bill you for.
Then if I hate the service, which I never have, I cut or eliminate the trip. If it is outstanding service, I will add to it, which I usually do. The sad thing is, that every ship I have ever been on prohibits individual tipping. How many of us have given a nice tip to the boat, but would have loved to do more for that one person that really sttod out for us personally?
One last comment, I went on one ship where it was actually suggested that tips should be 20%. That to me is a bit excessive for a $2,500 trip.