I think it's best if you just ignore it. These things tend to fix themselves once you forget about it/get used to it.
Probably some irritation problem. This thing occurs when foreign chemicals enter the urinary tract causing irritation. It's normal at times. Since you mentioned that you stayed in water for so long, that could be the case. The chance of something getting in increases when you pee while immersed in the water. Chemicals do not only come from pools (chlorine). Usually soap/detergent is the main culprit.
Could also be infection. If it's bacteria/fungal infection, then good luck. Usually such infection comes with pus and discharge. If you don't show such symptom (yet), then don't worry first.
At such young age (I presume), chances of getting cancer or relevant stuff, are very low, though still possible.
And as mentioned, if symptom persists, seek a damn doctor. Wtf.