Would you actually be better off in a small boat
The force of gravity is constant for all things. The force of a wave is equal on both the ships because the mass of the water
is almost infinite compared to either vessel. The velocity you reach flying into the immovable object is measured as 1/2 * Acceleration * Time squared.
Basically the longer you are in a free fall, the faster you smack into an immovable object. Jumping off a high building vs. falling down the companionway
. Same gravity acceleration but longer time yields faster terminal velocity.
In one case where your ship falls off a wave, you hit bottom it really won't matter if you are secured. The sudden deceleration can cause bodily internal organs to have a different velocity inside you when your body stops. Being secured has limits probably equal in both boats.
When the boat pitches you travel faster than the boat if not secured and smack into something sooner on the small boat than across the large open saloon
on a big ship. It's the relative velocity difference between the boat and your body that gives the little boat the advantage but only for that situation.
It's the relative difference in velocity that kills you in a plane crash unless parts
of the plane break off and have a relative difference in velocity. Thus being struck by a slow moving fork lift
has serious effects but a grapefruit just hurts when you are stationary.
Being strapped in saves you a lot of potential trouble but it is only effective if everything else is strapped down too.