In cold weather you don't have time to pull out life vests, throw float etc so they should be at the ready as best as you can. Also 25ft of unwrapped cheap (nylon) floating rope tied to your throw float. The rope should float so anyone who is in that cold water can swim/thrash to the rope cause they will never get to a float. It will keep being pushed away as you try to swim towards it in panic mode. The rope you can get to and pull the float to yourself.
Put/stuff a micro fiber around your neck area to keep water and cold air from going down the gap between your chin/neck. Really does make a difference.
Extra gloves.
A solar blanked to wrap around anyone that requires warmth.
Bag of play sand (in truck) to use at the ramp for traction. You can get a ticket for using salt in some areas. Been there.
Hot hands for wrists and feet.
Plug livewell openings on the transom to keep water from entering the hoses/livewell.... Good place to store stuff you require fast access to.
Food for carbohydrates to help the body warm up.
If you have an open boat. Motorcycle helmets or snowmobile helmets really help to keep you warm while running.
Take your time while fishing! Accidents happen more often when in a rush.
If you have a small cooler. You can preheat some aluminum foil wrapped bricks/rocks in your oven to 350*. Place those into the cooler with towels to take up all air space. They will last for a 4 to 5 hr fishing trip to have something really warm to hold to warm your hands if/when required quickly. Safer than taking a heater.
Battery jump box
That is off the top of my head.
Hope it helps.