[Humabout]

First: Question. By Jetski, do you mean the type you stand up on or waverunners (that you sit on like a motorcycle)?

Having ridden jetskis and driven small motorboats, Id day that a waverunner /could/ be a familiarity penalty on Motorboat. A jetski is likely its own specialty that has a default to motoroboat because of stuff like balancing involved.