I haven't been on a cruise, but the odds of that happening to you are very, very, slim. Think of the thousands and thousands of people who are on cruises this very minute. Outside of people cruising on very small charter boats, or cruising in the area of Samali Pirates, I assure you the vast majority of them will be OK tommorrow, except for stomach viruses and sea sickness, and maybe a hangover or two thousand. I would think with impending nuptuals, these type of events should be the least of your worries (See below for where I am coming from here). Just advice, but do want you both feel led to do, and allocate your energy to other matters. I could die at WDW next year, or be on a boat that wrecks there too, but that won't stop me from going.
I hope someone can give you good advice regarding the other part of your question. My daughter just got married in July. I am obviously a Disney freak, but I told them I thought Carnival would be better for a first-timer, and a honeymooner, more couple oriented. Of course, I don't know first hand though. I want to cruise next year myself, and I will go DCL if I go at all.