Can you use a regular tampon in heavy flow for an hour of swimming URGENT?

As long as your flow is sufficient for the absorbency, yes.

General rule of thumb - use lowest absorbency; if the tampon leaks within 1-2 hours use the next absorbency up, if the tampon is not saturated with blood after 4-6 hours use lower absorbency or switch to pads. Even if the tampon is a lower absorbency than needed chances are you will not bleed through within just one hour.