Deep cycle will definitely handle flattening and charging better than a standard battery. Do not think however it is best to flatten it before charging. Deep cycle lead acid batteries have the number of available cycles much reduced by running till flat. Best to get a bigger battery than required aand try not to discharge it much beyond 50% for a longer life.
I run a Minn Kota fresh water in the salt. My mate uses a motorguide fresh water model in salt as well. There is little difference between the two. Remember that water of any salinity is not good for electric motors. Just ensure you wash them after use, give a spray of Lanox or similar and keep the water out of the head unit you will probably find you get quite good life out the fresh water models in salt.
I wouldnt be surprised if the Kmart models are actually Motorguides. The older models with the extendable handle were being flogged off on Ebay for a while. There may be more that have be relabelled for Kmart. Close inspection will reveal all.