Hang out Poolside yes but it can be breezy & of course arid so getting out of a pool wet can get chilly quick. We rarely swam in October even though we had a pool in the backyard but the hot tub was definitely used.
The beginning of October is usually pretty warm still. I would say you have a 75% chance of getting pool weather. We have a pool at our place and it gets used pretty regularly from March-the end of October.
Bliss--DH and I have always joked that Halloween seems to be the start of "cold season" in Vegas. It's always shorts weather until that weekend hits, for some strange reason. Many pools will remain open through the end of October, but I'd say it will likely be too cold to lounge poolside at that point.
October is one of my favorite months in Vegas. Not too cold but not blistering hot like May-August.
Wet Republic will probably still be open. I think that's their last weekend of business. You should go.
ETA: I assume you mean the weekend before Halloween not the weekend after (which is in November). The weekend before it will definitely be open. I just confirmed that it was opened that weekend last year.
We're actually going down October 30 after work and coming back November 1. Maybe 2nd depending on how many people end up coming. So far it's just me and boo boo.
Halloween is the "start of cold" for us here in Vegas. It was really frustrating as a kid b/c you'd pick out an awesome costume and then, boom, that day the temp would drop and you'd have to wear a lame jacket it.