Dondra Lighthouse is a pleasant hour's drive S of Galle along the coast road. You can go up to the top of the light and walk around outside for great views of the seas. Some other reviewers mention in passing that it's neat the southernmost point in Sri Lanka. However, all the other reviews on TA don't mention the #1 reason I went to see it, So to speak, they may have missed the point.
Just as arbitrary lines on maps mark the borders of countries, other lines on charts mark the arbitrary boundaries of seas. The southernmost point of Dondra marks exactly where major seas come together. The ocean rolls on heedless of the boundaries, just as the Alps themselves don't care whether we call them Swiss, French or Italian; but the borders are quite official. The IHO, the International Hydrographic Organization, agrees on the lines, and they agreed the three major bodies of water meet at a single point, "the Southern extreme of Dondra Point. If stand at this point and look due south, you're looking at the Indian Ocean. But from the same point headed west, you see the Laccadive Sea. And from the same point an imaginary line running East by South marks the Southern boundary of the Bay of Bengal.
Technically you can jump into the Bay of Bengal, swim across the Indian Ocean all the way to the Laccadive Sea and swim back again in under five minutes. Better be a good swimmer though, depending on the conditions. Today there were heavy waves coming in from the west, and quite a strong current running from west to east; it was like swimming in a river. Rip tides are a real danger in Sri Lanka; if in doubt, better to err on the side of caution.
Naturally, all three bodies of water can be seen from the top of the lighthouse. I will try to attach two pix. The first was taken looking NW; the water in the foreground is officially the Bay of Bengal, while the water on the far side of the point is the Laccadive Sea. The second pic shows the point itself, where all three meet.
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