Here is the official text from the WI Statutes... The pdf isn't wrong, just very poorly worded. That entire structure applies to MINORS only, per the title of the section. Once an individual turns 18, none of those items apply. To the OP, at least according to the law, your 18 year old daughter is 100% legal, as she is not a minor and the laws in the section below do not apply...
The pdf handles this by saying it it is illegal for a parent to allow a child to operate unless these items are met. In the normal sense, parent->child, age doesn't matter. In a legal sense, child is <18 (minor), adult is 18+. They chose to use the legal definition of child, while most people would assume it implies family status.
http://www.legis.state.wi.us/statutes/Stat0030.pdf
30.678 Boating safety certificates; requirements;
exemptions;
operation by minors. (1) MOTORBOATS
OTHER THAN PERSONALWATERCRAFT. (a) No person under 10 years
of age may operate a motorboat.
(b) No person who is at least 10 years of age but under 12 years
of age may operate a motorboat unless he or she is accompanied
in the motorboat by a parent or guardian or by a person who is at
least 18 years of age and who is designated by a parent or guardian.
(c) No person who is at least 12 years of age but under 16 years
of age may operate a motorboat unless one of the following
applies:
1. The person holds a valid boating safety certificate issued
under s. 30.74 (1) (a) or a valid boating safety certificate that is
honored under s. 30.74 (1) (c).
2. The person is accompanied in the motorboat by a parent or
guardian or by a person who is at least 18 years of age and who is
designated by a parent or guardian.
(d) No person who was born on or after January 1, 1989, and
who is 16 years of age or older may operate a motorboat unless the
person holds a valid safety certificate issued under s. 30.74 (1) (a)
or a valid boating safety certificate that is honored under s. 30.74