I'm honestly confused why so many people seem set on comparing sea and lan beast on the assumption that you'll still be having most of your adventuring day on land. If that's the assumption you're running under, why take the sea beast at all. If you're in an underwater campaign, sea beast compares quite well to land beast on land. It's intended for certain situations. I don't see that it needs to be competitive outside its niche. Giving it a fly speed would be odd, wouldn't it. But the arguments don't seem to compare how bad a land beast is a hitting a creature flying several feet up compared to a beast of the air.