There's no doubt that Soviet Union had a special alliance with Cuba, but how did the supplies, people, and boats reached the Caribbean and essentially travel almost halfway around the world? Did they go through the Panama channel, no questions asked, or by reaching the southernmost tip of South America then go northwards? I assume they could also fly, but what about nautical travel?
I recalled this question from a few days ago when I saw /u/yodatsracist 's great answer here which provides a great explanation for how the USSR (and Russia) is able to gain passage from the Black Sea to the Med, and thus access to the Atlantic. I would simply append onto Yodat's that similar access from the Baltic is made possible via convention that makes the Danish Straits similarly an international waterway, which is guaranteed by multiple international agreements.