Bacillus thuringensis (BT) is an alternative worth considering. It's a bacteria which grows in the gut of caterpillars resulting in mortality, and is an approved treatment in many organic farming systems and urban forestry. It's a bit challenging to use, because environmental conditions for application are specific and BT can be ineffective if applied under poor conditions.
I have to doubt Sevin and swimming pools has been studied anywhere! If it has, I most certainly defer.
Sevin is carbaryl, a carbamate pesticide, which is mostly carbon, hydrogen, oxygen and a bit of nitrogen. I don't believe any negative effects on the pool water would occur from overspray. But like any pesticide, I would apply it only in accordance with the directions, including all recommended precautions.
A reasonable precaution would be to close the pool temporarily. If you have a cover, you could put it on the pool and hose it off afterwards. Remove the dead caterpillars when they fall in the pool. Carbaryl has a reasonably short half life in aquatic systems, generally longer in dirty water, but the breakdown mechanism is primarily by bacteria which would likely not be in abundance in a pool. Carbaryl is rated as moderately to very toxic, and dilution would be significant, but it is not possible to comment about effects. Carbaryl can be absorbed through the skin, which indicates caution.
Note that honeybees and bumblebees are very sensitive to carbaryl, so apply when temperatures are below 55F or over 90F when bees are less active. I believe the label includes more complete advice as part of the precautions.
From personal experience, individual smaller trees and shrubs can be protected with a bran bait laced with sevin, which reduces risk to non-target insects.
Here is a link to more complete information about carbaryl:
Carbaryl
