When sprinting (to start swimming) while ascending water, sometimes the player randomly will not be able to swim up... they will be attached to the ground and can move around but cannot ascend, which is dangerous in the case that you are drowning. The same thing happens when you are in lava too: You try to sprint to move faster and you end up swimming in the lava. But lava is a special case because
-You shouldn't be able to swim in it at all
-Placing blocks gets displaced as if you standing (so blocks are highlighted x placed higher up than the camera x crosshair should depict)
-You are essentially immobilized and every single time you try to unsprint to resprint, it makes you swim (and get stuck again) which is obviously dangerous when time is of the essence as you're getting baked alive in lava.