The differences between beasts and kings are not entirely clear yet but the largest difference appears to be just that, size. And while we don't know the nomenclature of sea kings, sea beasts typically seem to be 'Sea (insert animal name)'.
Sea beasts are big but not to the extent of sea kings.
Sea beasts, like their name, typically appear largely based on a particular animal with the head most closely resembling a land or other creature. In the case of Lord of the Coast, probably an eel sea beast? Which is pretty lame for sea beasts, but, that's the point I suppose.
Sea kings can also resemble terrestrial or other creatures but their design is much more free. Instead of being a fish/frog, it'll actually be a giant frog.
I needed to confirm the difference some time ago for some work and he definitely wasn't messing around but I always defer to his ability to change his mind.
I had to confirm a myriad of other things too but uh, yeah. We'll talk more about that in the future. For now, def a sea beast as per Oda's word.
In chapter 92, Arlong's eyes change when he snaps and Sanji notes that "They're the same eyes that a sea king has when it goes berserk!" combined with an image of the Lord of the Coast.