With respect to the Valkyrie comparison:
- Valkyrie is
much faster compared with both the Surfboard and the units it transports
- The kinds of terrain you want Valkyries to transport over is far more common than the situations you want a Surfboard for
- Valkyrie can be built anywhere and get to where you need it, Surfboard has to be built in the body of water you need it
- Valkyrie is much more aggressively costed (80m vs. 220m)
Surfboard may need buffs and fixes in addition to this to be viable. In fact, it probably does.
However, if you have to make a Ship factory to make Surfboards, and there is enough water that this seems worthwhile, I have never seen a situation in which making Surfboards (even if they were buffed) was preferable to just making combat ships.
Moving ground units around, on the other hand, is frequently preferable to making gunships - or at least useful in combination.
I don't think limiting it to Athena is necessary - Athena is already pretty niche in its own right.
This might have the (partially) unintentional side effect of allowing landfac cons to take sea mexes. I don't see this as a huge problem.