46 CFR § 69.171
When the tonnage mark is considered submerged
April 15, 2021
For the purpose of this subpart, a tonnage mark is considered submerged when—
(a) In salt or brackish water, the upper edge of the tonnage mark is submerged; and
(b) In fresh or tropical water, the upper edge of the line for fresh and tropical waters is submerged.