This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
艠 is a rare character combining 舟 (boat) and 登 (ascend). Its historical development is uncertain, and it may be a kokuji or a specialized variant.
A boat (舟) that climbs (登) waves: imagine a vessel ascending a swell, but its exact meaning remains elusive.
For トウ, picture a boat ascending a tall wave, reaching the top (トウ) of the swell: top -> トウ.