We had a dachshund that loved to go boating. He was a regular
shorthair, and a real trooper. He went out every weekend with Dad on
the ChrisCraft cabin cruiser.
The narrow sides (gunwales the right name) on either side were no
problem for a small narrow dog like him. He'd wander the decks like a
Bosuns mate, looking everything over and giving anything that came
close the once over.

On a side note...
The weekend my Dad died on the boat from carbon monoxide poisoning,
the dog proved to be an interesting puzzle.
We wondered if Dad had died from other causes at first since the dog,
Baron, was found in good health.
The sheriff had to trap Baron in the head, since the dog was NOT -
GONNA - LET - YOU - NEAR - THE MASTER! down in the cabin.
Those of you that have owned a dachshund know what I mean when they
switch from happy dog to business mode.
We finally figured that he had taken himself to the shore frequently
enough that day, that he had not got enough CO to build up and kill
him.

One time we were in our runabout and the Red Baron von Schlitzenheimer
was so excited to get ashore, he leapt a bit too early and missed the
dock.

Dachshunds can swim, yes, if they walk into the water from the shore.
Leaping in...nope... I dove deep below the dog, as my Mom leapt in
nearby.
It was funny, watching the dog corkscrew backwards down into the
depths, paddling like mad. I shot back up with the dog and he was
none the worse the wear from the experience, although he learned from
that day to ...wait for the boat to stop. Smarter than some people in
that area.

We had 3 different dogs on numerous trips and never had motion-sick
dogs... But that dachshund road in the car, the boat, and Dad's
Beechcraft Bonanza too.