The only way to prevent them is good construction and a solid base. If the shell flexes at all under the weight of the water it will crack the acrylic, which is almost paper thin. If you notice any deteriorating wood behind the cabinet or settling on whatever base it's on, repairs to both will prevent future damage.
There are various polishes that you can use to try to eliminate the visibility of the cracks. If you get a bad one it's a good idea to drill very small, very very shallow (like 1/32") holes at either end to prevent it from getting worse.
Thanks. The spa is attached to the pool and set in cement with pavers all around it. The only place I have cracks is on the small lip that sticks out around the tub. Ill post a pic.
I have a lot of kids that are always in there and jumping into the pool off the edges and stuff. That can't be helping
What polish should I go with?