swimming stopped after a duration

I'm on a Galaxy Tab A.

I came out (going up) from an underwater cave, and rose to the surface with no problems.

I looked around and picked a nearby island. I was able to swim very close but then started sinking without being able to correct it. I was still moving forward so collided with the island underwater. Nothing I could do would allow me to rise, so ended up drowning. Is there some kind of time limit for swimming?