How to reproduce:

1. Start fast swimming ("horizontally")
2. Swim into horizontal 1x1 corridor through flowing water
3. Reach end of the water flow range, remain in 1x1 corridor
4. You are crawling now, but your model is standing. Your head is literally inside the upper block, but you are not "drowning". Camera is on your feet level.

Possible solution:

The only reasonable thing I can imagine is introduction of legit crawling. There is another idea to block fast swimming in moving water, but it won't solve the problem at all - someone may drop sand on you, place cobble over the sand and then dig the sand or something like that.