I have a Venture Kayak Easty 15 and it has developed a flat spot on the bottom center just below the foot rests. What is the best method to return the bottom of this kayak back to its original convex form?
It sounds like you have oil canning. It's a standard problem with plastic kayaks, usually caused by excessive tension in straps when loading onto roof bars or long term storage on bars.
Various solutions exist, mostly they involve heating the affected area and putting pressure on the inside to restore the shape. You can use a heat lamp or a bright sunny day as a heat source but make sure you don't leave it unattended, especially with artificial heat sources, as letting it get too hot can ruin the boat.
Unless it’s so asymmetrical that it’s noticably making you turn I’d strongly suggest not worrying about it. At recreational kayak speeds I doubt it’s adding measurable drag, especially with a rockered hull like that which is made to behave itself in a variety of water not set flat-water speed records. The great joy in a boat like that is you just paddle it you don’t have to fuss over it—the oilcanning, or the scratches either.
If you’re determined to fix it first just try leaving it belly up in the hot sun for a day and see if it fixes itself (don’t store it that way for years on end sunlight is the enemy of plastic boats). Another option is lay it in soft sand on the beach on a warm day, hull right side up, supported evenly with sand, then dig a hole under the flat part, fill the interior of the hull over the flat section with enough warm sand to gently encourage it back into shape (use plastic bags if you don’t want the mess) and let it sit that way, ideally until the sun goes down.