So I think Vikings in general are not the most interesting to play in the very late game, but I want to focus on the trade off for them getting the Longboat: lacking fire ships entirely. They are the only civilisation in this situation, and in general, that’s totally fair, as they can do what other civs can, but better.
However, against Turtle Ships, Thirisadais (especially now that they both get more range) and Caravals, the Vikings are basically powerless, and arguably one of the worst civs on water. To some level, microing can alleviate that, but I think it is far from fair.
My suggestion, at least for playing versus Koreans and Dravidians: give Longboats/Elite +1/+2 against “Turtle Ship” armour class, like fire ships.