There are a couple of options.
You didn’t say what country you’re in, and that does make a difference, so I’m going to assume that it’s the US, but if it isn’t, speak up. 
Anyway, I think one of the most popular options for a light switch is to just use one of the in wall Micros. This can be placed anywhere along the circuit branch. And then it can be used with the existing switch in most cases. So that’s often the simplest solution.
However, before we go any further it’s really important to ask about the specifications of the particular switch you want to make smart. Your topic title said “outdoor light switch,“ but your post said “pool filter,“ and those are usually very different specifications. Since this is for a pool filter, those are often not your standard 120 V 15 amp switches. If this is for either a 220 V line or a 30 or 40 amp switch, that’s going to make a huge difference.
There are several outdoor rated options that work for water heaters or pool equipment or other higher amp equipment. Some are added in line so again you continue to use the existing switch, and some are replacement switches. See the following discussion thread:
So anyway, there’s probably a good solution, but we do need to know a few more details. . The brand and model number of the pool filter would help, too.