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The Yucatan Peninsula is entirely in southern Mexico.
The Yucatan Peninsula juts out between the Caribbean Sea and Gulf of Mexico. So this also means that the channel connects the two together.
http://en.wikipedia.org/wiki/Yucat%C3%A1n_Channel
That places you on the Yucatan peninsula - close to the city of Merida.
The Yucatan Peninsula is on the south side of the Gulf of Mexico. 18.8067° N, 89.3985° W
43' n