Your pool deaths are inflated. The 390 is over 3 years 07-09. And gun deaths spike after age 14 where I presume pool drowning a continue to fall. Further, guns lead to very large injury numbers which I'm guessing greatly exceed pool-related injuries even more. Pools aren't that dangerous.
The 390 is the annual average number for those 3 years, not total. According to wonder.cdc.gov, for ages 0-14, there were 334 pool deaths in 2008 alone.
You're right on the pool death numbers being an average. But I'm still not sure you're point has been made.
I wonder what the statistics look like when taking into account injuries too.
You can't get very injured in a pool (I guess you can slip outside the pool - but you can slip anywhere playing so does that count?). But there are probably more firearms injuries than deaths.
Using Consumer Product Safety Commission 2008 numbers for drowning, and the anti-gun group Children's Defense fund numbers for firearm deaths.
http://www.cpsc.gov/en/Newsroom/News-Releases/2012/New-CPSC-...
http://www.childrensdefense.org/child-research-data-publicat...