Other Dan is right, there's probably no definitive rule. But most of the time, I'd say: SEAs require articles (THE), pretty much always. I can't think of any example of using 'sea' without 'the'. LAKEs generally don't require articles. We took a cruise on the Mediterranean Sea, the Carribean Sea, the Sea of Japan, etc. etc. We took a cruise on Lake Geneva, Lake Baikal, Lake Superior, etc. etc.

Sasha Generally, with names of individual lakes, we don't use the for example Lake Victoria (not the Lake Victoria) Lake Baikal (not the Lake Baikal) Lake Michigan, Lake Tanganyika, etc. But when the word "lake" comes after the name, like "the Lake of Baikal," it’s more poetic or formal and less common in everyday use. Now, when you're talking about groups of lakes or chains, it's more common to use "the," because you're referring to a collection: The Great Lakes The Shatski Lakes The Finger Lakes As for "the Dead Sea" — yes, despite being a salt lake, it's called a "sea," and for seas, we do use "the": The Red Sea The Dead Sea The Black Sea Hope this helps

It's not so black & white, it's a spectrum that depends on many factors. The same pattern applies not only to lakes, but to roads, to cities, and many other things. Factors include: Does the official name use the word "the", and did it ever? This is the most obvious factor, and by far the main thing to consider in formal literature. Does the proper noun sound like a common noun? If it does, it's easier to use "the" when it's not strictly part of the proper noun. Do people like to talk affectionately about it? If it's something grand or beloved, people often add 'the' when it's not gramatically necessary or expected. So basically, you juggle a whole bunch of factors, including what sentiment you wish to convey, then you make a decision how you want to speak. It's not a strict science at this level, it's an art.