Probably 50% to 70% of my wedding photography was done with a 50 mm lens. I shifted to a 35 mm lens for groups. Rarely would I use a 90 or 135 mm lens—maybe one shot during a church ceremony. Rarely would I use a 28 or wider.

Your zoom is fine, keep it handy, but do most of your work with your 50 mm lens, and your feet. Step back about 18 feet to get a full length photo of the bride and her dress, with the train. Step closer for tighter shots.

Shoot verticals for singles, couples and threes. Close shots of couples are horizontals. Anything over 6 people is probably going to be a horizontal. Between 3 and 6 is a toss up. Depends on the pose.

Weddings on boats can be very pretty. And, Nobody leaves early!

If you are between decks you’ll have a low ceiling, which works well with bounce light. If you are on an open area you may be under a canvas tent-like cover, which is iffy with bounce flash, and of course if you are under the open sky you need direct flash.

You only get great sunset lighting for 10-15 minutes. If you use that for a background, even a small flash will provide fill-in light for detail in people’s faces.

You can practice fill flash techniques ( best sync speed, distance, etc) at home with volunteers (or conscripts) of family and friends. Haul out the troupe as the sun approaches the horizon and check your watch for the time. After the sun is down you’ll have a few minutes of afterglow when the clouds may be stunning. Then check your watch again. See how much time you actually have. Count how many pictures and poses you could make in that time.

On sunset cruises, get as many groups as possible before sunset. Concentrate on the bride, groom, their families, and the bridal party at sunset. After that all your light will be with flash. Bring extra batteries. Ask ahead about when the boat is scheduled to return. That will give you an idea of how many hours of darkness to plan on. Open decks, with no walls to reflect the light, suck the light (and battery life) from flash units.

Don’t expect to find a wall socket for recharging anything. They might be available, but they can also be unreliable. Obviously the boat is not connected to the grid. It has its own generator, and voltage and frequency can vary widely from what your equipment needs.

The cruise boats typically have plenty of lights, but they may be more ‘mood lighting’ than portrait quality illumination. The faster f/1.8 50 mm lens may be easier to use.

Bring a sweater. Even if the day is very warm the evening will cool off fast on a boat. You want to be comfortable.

I can’t think of a reason to bring a tripod.

Bring a few plastic bags. Who knows what the weather will be like. A stiff wind can create a bumpy ride, or a spray. If so, you can shield the camera until you pull it out to take a picture.

Finally, bring a towel.

Enjoy the wedding.

I had over 30 years of professional wedding photography business. You will be on a confined space on a boat, at sunset you will have to shoot fast and you will not have time to switch of lenses, therefore it is my professional opinion that you only use the 24-70, since you will be balancing flash with the sunset an f4 lens will not be a concern.
Personally, if I were you, I would be practicing taking as many shots as I could at sunset on anyone who will pose for you so you can be confident about your balanced fill flash at sunset. So once the wedding day comes, you will be ready.