The reality of wet bags is this. You can only protect them so much. Theres a couple of different factors that come into play, whether you are at a port, airport or somewhere in between.
First, is whos doing the loading? Are they the 9.00 low bid or someone that actually cares about their job?
What kind of equipment are they using. When I worked at JFK, there was a lot of baggage carts that were trashed, the doors didn't close right, holes in the roof, you name it they had it. A lot of the baggage cans, had holes in them and looked like swiss cheese. They were supposed to put plastic in first, then over the tops, but that almost never happened. Some of the new terminals that opened before I moved, had brand new equipment and crews that actually cared.
What stage are they in loading luggage/passengers? If its early, they may still be concentrating on getting everyone out of the terminal and off the ship, and may not have hands to move incoming luggage. The last cruise we were on I happened to overhear that some stevedores were going for breakfast and taking orders for those that couldn't leave, so that's something else to consider.
How suddenly did it start raining? This is Florida. One of the only states rains storms can sneak up on you. 1 minute its nice and next its a 30 degree temp drop and raining sideways with 60 mph winds. 10 minutes later the suns out like nothing ever happened. Usually they are pretty attentive but everyone has bad days. Some more then others.
With that said, accidents still happen. Ive personally had damp clothes from an airline flight. A lot of times what happens is that a seal on the baggage door fails, and the belly becomes unpressurized. That a lot of times leads to condensation and depending on the flight, can seem like it rained on your bags. Ive been in several 747 bellies that were still dripping after being on the ground for 2 hours.
Its also not uncommon for bags to end up off the pier. Bag belts break, carts aren't lined up properly, bags are loaded too high, things happen. I forget what line off hand but I remember 1 ship had a whole cart get dropped into the water pierside, with all of its bags. Passengers were taken out 1 by 1 to claim the bags by crew members. Another time, and it might have been the Wonder, had a waste oil truck hose break spraying down several carts of luggage that were staged to be loaded. That was another multi passenger page.
No matter what luggage you buy, you are at the mercy of who and what is being loaded. All it takes is for someone to throw a heavy bag on top of yours and its game over.