I've got these 2 giant apes like creatures who are too heavy to go on the ship to be taken to another continent. What are some ways that they could possibly get across the sea? This is a medieval epic fantasy so I can't really fly them etc.. My apes are 12 feet tall and weigh 18,000 kg. They have to travel 510.8 km oversea, going from a desert like climate to and normal spring like one.
Ancient undersea tunnels.
In your world, there are learned individuals who are aware that there exist tunnels built by a prior civilization. Half-drunken intrepids claim to have used these tunnels to traverse the oceans by travelling beneath them, and one of these was in possession of a crude map. Some of the symbols scrawled on this map defy explanation but the gist is clear: the tunnels cross the ocean.
This is how you move your apes. They travel willingly on foot, comfortable with their keepers and the treats they are given as inducement.
Everyone knows that ancient tunnels are full of monsters. And the only thing better than ancient tunnel monsters is having giant apes show up to battle them!
Chain of islands
Cuba is 1000 km from South America, and Hispanola is 600 km. Yet, both of these islands got giant ground sloths from South America in the Oligiocene, long before the sloths got to North America (which is closer to Cuba).
How did they get to these distance islands?
Lots of little islands! Going 500 km in one swim is a tough ask for a land creature, but a bunch of smaller hops is more obviously feasible.
While the sloths that actually spread to cuba were smaller, some ground sloths were very large and could stand 12 feet tall, though not as heavy (although 18 tons is very very large for a land animal...)
It has been estimated, using the square cube law that a 24 foot 1930s King Kong would weigh around 15,500 kg. Your ape is 12 feet tall and weighs 18,000 kg.
Clearly your apes are either extremely obese or extremely dense (in terms of mass not intelligence!).
If they are obese to that extent then they aren't going to be able to move at all. I suggest you simply immobilise their arms and tow them to wherever you are going. They'll float easily with a harness fitted with floats and weights to keep them the right way up.They may hate it but what are they going to do?
If they are extremely dense then no wonder they don't like water - they would sink immediately! This also explains why boats aren't a good idea. If they move around too much they're going to tip most medieval craft over.
If they are denser than water and not allowed to go on boats, the only option is to provide some sort of breathing apparatus and walk them along the sea-bed. Blindfold them and accustom them to the water gradually.
This is rather unlikely but if they are intelligent enough to learn, it might just be possible. They could rest at night by floating. A trireme shouldn't have too much trouble pulling them at a decent speed. https://youtu.be/e131NJWTeuE?t=36
When I have some time I'll do the actual square-cube law calculations. There is a likelihood that the body structure won't be able to support them unless they have a special kind of muscle tissue hitherto unknown.
Square–cube law This principle states that, as a shape grows in size, its volume grows faster than its surface area. When applied to the real world this principle has many implications which are important in fields ranging from mechanical engineering to biomechanics. It helps explain phenomena including why large mammals like elephants have a harder time cooling themselves than small ones like mice, and why building taller and taller skyscrapers is increasingly difficult.