Depending on your type of tire there are several things that can happen. I ran radial tires for a few years and had to replace them often because the belts in the tires would break. If your boat sits for long periods of time and then you start pulling it at speed then you can snap the belts in your tires from the "flat spot" that it was sitting on.

I changed to 16 ply bias tires and never had the problem again. Ran the same tires for five years.

If you are running radial tires you should start the tow slow or move the boat to a diffrent location for a few hours before you start pulling so your tires will round back out.

If you watch your tires as you start to drive and you see them hopping a little and then it smothes out your tires have flat spotted.

just my experience with tires on my boat.