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.

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.