Does a Thunder Bay marine engineer qualify for TN when the American work only happens during winter lay-up?

Seasonality is not itself an obstacle. Eero satisfies the Engineer entry through his degree and provincial licence, and Canadian citizenship covers the personal half of the test. What still has to be shown is that the American position consists of professional engineering during the period he asks for, and that he is entering as an employee rather than trading on his own account.

The person and the position are decided independently

Two separate findings are needed and neither rescues the other. The applicant must be a Canadian or Mexican citizen and must meet the credential rule attached to his own profession, which for Engineer is a baccalaureate degree or state or provincial licensure. Then the American position itself must require and consist of that professional work.

A lay-up assignment usually has no difficulty with the second finding, because specifying a shaft alignment or approving a hull repair is engineering by any reading. Difficulty arrives when the same eleven weeks are also expected to cover crew scheduling, purchasing and general yard supervision, because those activities belong to a production role rather than to a profession. The way through is not creative drafting.

It is for the operator to decide what it actually needs an engineer for, and to describe that plainly enough that an officer can recognize it.