It depends on the length of the swim spa, the quality of the construction and the installation.
Bowing out is caused by the water pressure out and down. Most swim spas have a system of wooden ribs that are attached with fiberglass to the acrylic shell. These ribs support the shell and distribute the weight of the water to prevent the outward bowing. If the swim spa is not installed on a perfectly flat pad, then as the water is added the middle of the swim spa can bow down and this will force the sides to bow out (1/4 inch down can result in 1 1/2 inches out). To fix this the manufacturer will often suggest that during the installation while the swim spa is empty, shims are placed under the vertical ribs to ensure that the swim spa will not bow down when filled with water.
If done well, a 17 foot swim spa should bow out no more than 1/4 to 3/4 inch.