Pool lines, electric pool heaters and other pool accessories are good stuff and selling those can be profitable.
I actually had (still have) a website about Inground Pool slides. I built that site knowing that there are money in this business. That is one of the Adsense niche site that I built using the guide made popular by Xfactor (which reminds me, the latest Adsense course is still not available.. Come on John!).
Anyway, this post is a clue for all of you niche marketing. You can try and do some more a little digging and you can build a related site about Pool Heaters.