[PvK]

If that's what he's saying, it doesn't look to me like he is correct. None of my Oceanians have any penalties either on land or on sea (except that some can't come onto land at all, because they can't grow legs). The icon is misleading for the units that change shape when they come onto dry ground, but it has no actual negative effect. It's more like disinformation for the enemy, who might expect, say, a Wave Warrior, to be a poor fighter underwater, when they see his legged form on land - but they have no penalties on land, and when they go underwater, they change shape into a fishtailed shape that also has no penalty underwater.

PvK