Principal
Principal, head, foremost, major, dominant, focal, prime, premier, first, grand, supreme, (most) prominent, paramount, cardinal, vital, capital, salient, essential, intrinsic, chief, leading, most important, ruling, central, key, master, primary, high, fundamental, predominant, pre-eminent, overriding, crucial, critical, pivotal, elemental, staple, and urgent.
Sea
Sea, deep, the drink, ocean, brine, and the briny.