Is there any place in the US where there is good glider cross country
potential near a beach? *Trying to come up with a vacation spot that
meets the criteria of both of the people on the vacation.

Absolutely! And your timing would be perfect if you wanted to go soon
(before it gets hot there...) Dillingham Airfield, North Shore of
Ohahu, Hawaii. You didn't specify mainland, and I believe that's the
best beach soaring in USA. Talk to Steve Lowrey at Acroflight
International: http://www.acroflightinternational.com/ It's a
fantastic place to fly, and an easy sell to the Mrs(?) if you can
afford it. (note-nice $5/night campgound right across the street from
Dillingham in addition to standard options...) Worth consideration;
when I flew there I never needed more than 1000' (actually 800' but
they have a 1000' min charge...) since the Tradewinds are almost
always blowing at 10-15kts steady, perpendicular to a steep 1000' high
ridge that runs the length of Mokolia Point, about 3/4 mile south of
the (9000'!) runway that runs parallel to it. A few laps on the ridge
and you are high enough to thermal hunt in the pineapple fields and go
explore. People do fly X/C there despite the limited real estate,
although lazily drifting around in the convergence slightly off-shore
while watching pods of whales/dolphins with some acro sprinkled in has
it's own appeals too should you stay tethered.

More locally, if you can time it right, good ole' Torry Pines could
possibly fit the bill too. http://flytorrey.com/cms/

I can't speak for the X/C potentials of the FL sites, but Seminole is
not too far from the Pacific or Atlantic Oceans (but not a 'beach'
site) and being right next to Orlando can make the sell easier than
some other sites.
http://www.soarfl.com/main.htm and of course in Fl you also have
Miami Gliders: http://unusualattitudes.biz/ , not on the beach either
but closer to the water.

I vote for Hawaii!

-Paul