This is where the rock or platform comes in. They need someplace where they can get completely out of the water and bask under a heat lamp. You can purchase a rock platform at the pet store. I tried rocks from the back yard, but it was hard to get the right height in proportion to the water level. Turtles can gouge their shells on sharp rocks, yet they need something that their claws can grab onto so they can climb up.
Some people use floating logs, floating platforms, rocks piled up in a corner.
Use your judgment. With the addition of an under water heater you need to make sure the turtles won't be pushing rocks onto the heater or using the heater as a leg up onto a floating platform. I've found the stationary rock platform to be the best thing in our tank. It's a piece of rock-like material that is shaped like an H but is curved in a half circle. The turtles are able to go under it, and like to slide off the top as well.