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.
Thank you for your report!
We're actually already tracking this issue at MCPE-58953, so I will resolve and link this ticket as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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