If you haven’t already, get your dog or cat used to being around the water.
Introduce them to the boat (and boats noises) a little at a time.
The best way to introduce your pet to boating is to spend some time together on the boat when it’s tied up to the dock. Training goes something like this: An hour on the dock, a few hours and start the engine (to adapt them to the noise and vibration), a quick trip around the marina, a daysail, a weekend overnight, you get the idea…
Practice swimming and rescue drills with your active dog.
Train the dog to paddle to a swimming platform or ladder, so they could be helped back on board. Figure out your plan well in advance. “What would I do if. . .” Fill in the blank, then make a plan. Don’t forget to practice your drills on a nice day, when it’s “fun,” so everyone knows the procedure.