You cannot start swimming in water at a height of half a block (1/2 block), a block or a block and a half (3/4 block). Even when it is possible to swim in these water levels (when you are already swimming). 

Step to reproduce:

• Prepare three spaces to put the water and two of them should have a slab at the bottom.
• Fill the spaces with water (waterlogging the slabs) so that one has water half a block deep, another one block deep, and another one block and a half deep.
• Try to swim in each of the spaces with water.

Observed Results:

You will not be able to swim in any of the waters with height less than two blocks. 

Expected Results:

You should be able to start swimming at all water levels since (as shown in one of the videos) it is possible to swim in those water levels.