Many posts on this forum start with "...I am shooting a wedding..." and inquiring about... "which equipment to use" which indicates to me that these folks have little experience in wedding photography, let alone on a boat in what can be difficult sunset lighting conditions.

Off-camera lighting, as suggested, or multiple flash is great, provided you know how to employ those techniques. If that off-camer light or slave unit is not placed properly it can exacerbate an already difficult lighingt situation so you may be better off with a single on-camera Speedlight and the know-how as to controlling the ratio between the ambient and the flash illumination.

"Boat" is a very broad description as to size, length, capacity and stability when it is in motion. Hand-holding a camera on a wet deck can be tricky. Space can be an issue as to focal length. If space is limited, you might need a wide-angle focal length but that may not be the best choice for portrait-like shots- you may need to compromise and be very careful of distortion or foreshortening in full-length and group shots.

It's hard to give sound advice if I don't know your skill level, experience or knowledge of flash fill and multiple lighting techniques. If this kida work is not in your wheelhouse, and you still wanna do it, it might be best to keep things light-weight, portable, self-contained and simple rater that employing a flash technique that you have never attempted before.

More details might help as to what is expected of you, what you know about certain extreme available light conditions, and what you know about balancing electronic flash with late-day sunshine as to light output and colour temperature.

I don't wanna be discouraging but there are issues to think about!

Good luck!

I would say you are probably better off changing the coolant Bi-annually with all of the aluminum in the cooling system, But I have seen mid range trucks that were 15 yrs old and never had the coolant changed.
My Son-in-law has a 2010 Dodge Ram 3500 4X4, he was having brake issues with it, he took it to the dealer he had bought it from (used) They ask him when was the last time you changed the brake fluid? They recommend you change it annually and use DOT 5 fluid. They flushed his brake system with new fluid, everything was back to normal.
I retired from auto mechanics after 60+ years, three years ago, and this was the first time I had very heard of this.