I want to do a beach trip in October
 

somewhere on the east coast. How far south do you think I need to go in order to have swimmable water (temperature-wise)?

I would say your best bet would be Florida. Anywhere on the Florida coast will be enjoyable. I would recommend Cocoa Beach, Fl. Every trip I have taken there has been so wonderful. relaxing and close enough to Orlando to do all the tourist-y things.

Hillary
www.edufire.com

We went to Cocoa Beach last year the first week of October and the weather was great. The water temps were plenty warm. We stayed at the Resort on Cocoa Beach-all the condos have two bedrooms and two baths-and it is right on the beach.

We go to the Carolina beaches nearly every October and they are always swimmable. It takes much longer for the water to cool off than it does to heat up. October is one of the absolute best times of year to enjoy the Carolina beaches. www.ncbrunswick.com, www.visitnc.com. You can also probably find an entire cottage beachfront for what a tiny condo in Florida will cost. For water temps www.nodc.noaa.gov.

I really did not want to go as far south as Florida. North Carolina sounds a lot better. I would be driving with my family and it is much closer.

If you have already ruled out a location, it's really helpful to put that in your original post.

I will be swimming at Wrightsville Beach,NC in early October. I have often enjoyed Atlantic Beach, NC in October as well.
Another option would be Isle of Palms, SC.
Have fun.

I have not ruled Florida out if that's as far north as swimmable water exists that time of year. I was just hoping for something further north as I said in my OP "how far south do I need to go?"

I would tend to use Cape Hatteras as a cut-off point, as that is where the Gulf Stream comes on board, and any points below are always significantly warmer than those below. That said, it is completely accurate that the water is fairly slow to cool down, and I've been in the water as far north as Ocean City, MD in October (I wouldn't count on that though, as the air temperature may well not cooperate, and I'm a Jersey girl and used to chilly water). I have found the water in Myrtle Beach, SC to be pleasantly warm in October (better than the air temperature- we were running into the ocean to get warm).

You could also consider Hiltonhead. I would think that the water would still be pretty warm in October.

Sorry, that should read "points below are significantly warmer than points ABOVE". Oops!

Part of the answer depends on how warm you want the water to be. We've had friends from FL visit the Hamptons and they won;t go in the water even in Sept (when it's probably warmest) since the north atlantic is just too cold for them ever.