Can you float the river during March and still enjoy it or is too cold for the beginning of Spring? Are the water activities open on weekends during March/April?

The FRIO river is cold year round. You will enjoy it now, but will be chilly.

The water temperatures do not fluctuate much. Some shallow areas may warm up, a bit. The deciding factor won't be the temperature of the water, but of the air.

Frio River is cold year round

The water is always cold that's why you can see the rocks at the bottom

I would do it even tho it will be cold

Yes, the river temp is cool. However, very worth it