Years ago, around 15 or so, in New Orleans, the fuel truck had a coupling on the hose come off..1 or 2 cages full of luggage got soaked with diesel fuel. The passengers did not find out until after the cruise had set sail. There is no way you can wash/clean the luggage and the contents...
Luggage falling into the water, though a rare occurrence, still happens occasionally.
I bring on 1 medium size and 1 back pack...always carry on. When my husband used to cruise with me, we did 1 large, 1 medium, and tote. Always carried it on. The porters would tell you, as you passed by, that you would have to check it...but I know better..having done so many, many times.