We've taken Skyla, our Sheltie on various trips on "Cowal". She loves being with us on the boat, and sulks if she can't come
She wears a medium size pet life jacket, which has a lifting loop and D-ring built in. We loop her lead through a seat back, then clip it on to the D-ring. (The connecting clip should, I hope, break away if the boat went over - but I wouldn't want her [or us] out there if it's that bad!) She normally tucks herself in between the seat pods and watches the world go by. If it gets a bit lively she tends to lean against one of us - a hint to take it easy.
She's a 'clean' dog - let's us know when she needs a pee etc, but accidents can happen. Take a bucket and a brush for cleaning up.
Take some drinking water for it, and a snack if it's a longer trip.
Thing about getting on and off the boat: can you carry the dog up a ladder? Will it let you carry it up a ladder?
The most important thing is: Will the dog enjoy it? If not, please don't take it with you. If it does (and I've never yet met a terrier without a sense of adventure
), you won't be able to stop it!