Suppose we scale up a human to about 10m in height and their skeletal structure have evolved to ensure they won't succumb under their own weight. Due to their unique blood circulatory system they can remain conscious on land, however their weight would easily match those of the Titans in the prehistoric period. They aren't very intelligent so they don't build rafts, but I need them to swim across Lake Baikal. What other biological features must the giant have to become a swimmer?
Assuming they have the same mechanics as a human (which seems to be the case) they'll be able to accomplish basic stroke movements. Weight might be an issue, but the fix is easy; you just need to lower their density so it's less than water, and they'll naturally float. Alternately, just near the density of water and it'll be easy to stay above the water's surface.
Fat's density is about 910 kg per cubic meter. Density of water is 1000 kg per cubic meter. Assuming their biology as a whole is basically just a scaled up human, their density also should be basically just a scaled up human; this means being fat enough to more or less float (or at least not sink too quickly) is perfectly reasonable.
I'm not sure I see the problem. The density of a human remains the same if scaled evenly. Yes, there would be more bone (heavy), but also spread over a larger volume. Density is what allows something to float (less dense than water). Density is a factor of mass per volume. A perfect scaling would prevent the giant from succumbing to their own weight (look at elephants or dinosaurs - the bones are thicker to support the weight - but they still are roughly the same density), and would keep overall density without a pretty slim margin the same as for a normal-sized human.
Even if your giant is too dense to be able to swim, he could still make his way across the lake.
He simply knocks over a tree on one side of the lake, drags it to the water and uses the tree as a float as he kick-swims his way across.
If your giants regularly carry around a huge tree-sized wooden club (ie. a tree), then he can just use that as his floatation device.