sponge to improve water retention?

I live in very hot south Carolina and have a few raised beds. It takes lot of watering. I am planting a new bed of black krill tomatoes. Ian thinking of putting a sponge in the hole to absorb and hold water. Any thoughts?

The sponge will rot and fester with bacteria... not the good kind, either.  What sort of mulch are you sporting?

Soil organic matter is the sponge you are looking for. Add compost, leaves, manure, wood chips, branches, cardboard/newspaper, or whatever is available near you.

Do a search for hugelkulture - it's like a raised bed with a lump of wood, ideally a log, in it which absorbs water during rain and then releases it very very slowly.  In time the wood will rot down and feed the soil, too. 

Here's a link - http://www.richsoil.com/hugelkultur/

Well I have a mixture of shredded Bark and composted cow manure. Plants grow fine but l water daily.

I am thinking of putting a luffa sponge under plants on New bed. Small bed no room for logs

Ok, but one luffa sponge per plant is not going to hold much water, and seems like it is not the highest use of that resource.

Which part of Carolina you in? What is your soil like? I lived in the sand hills outside Columbia for a while, and the soil there (not surprisingly) was sandy. Watering everyday in the heat of the summer might be needed for shallow rooted plants in the sun ... trees make better use of the water cause their root system goes deeper, they can get to water and minerals that garden and field crops cannot.

look into terra preta soils, and do you mulch?

I am in Columbia. Goal is to water tomatoes every other day. I have a layer of wheat straw on top of soil

You need mulch. Thick mulch.

I met some people in sandy Florida that completely redid their yard in permaculture. Many of their plants wilt during the heat of the day, but pick back up overnight and look fine each morning. If this is the case, you could easily go every other day with the watering.  My tomatoes that are in pots will not make it more than a day during the worst of summer (big plants, small root system), but the ones that are in the ground fare ok with alternate day watering (and less). And it is so hot now that I have proclaimed tomato season to be over for me ... the new fruits are getting pretty rare, and they don't mature to full size. Maybe your black krill variety does better, but most types simply fade away in the full heat of summer in the south.

I want to clarify my earlier post, because it sounds a bit too abrupt, but I think a thick mulch is especially useful when it comes to hot weather and tomatoes. I'm talking like 3-6 inches depending on how bad the weather is and what your soil is like. It doesn't matter to tomatoes since they like to root everywhere and heat and dryness will guide their rooting behavior. They naturally like to dig for water.

You might also look into wicking beds for the future:

http://www.instructables.com/id/Wicking-Beds/