A water heater is the same type you'd use in a fish aquarium. A lot of us, myself included, use the Visi-therm stealth heaters, they are fully submersible and nearly indestructible.
You really need to make a number of important changes. I know you just got these turtles but their tank size is totally inadequate for the size of turtles you have. You can also get a rubbermaid/sterlite container.. and use that as temporary housing. It is recommended that you get a tank/container the equivalent of 10 gallons per inch of turtle shell.. (measurement minus the curve).. that would mean you'd need at least an 80 gallon tank/container. You really need a filter. Turtles are very messy .. we recommend a filter that is rated at least 3 times the tank size. This means that most filters are rated for fish tanks which don't have nearly the amount of waste that a turtle tank does. so therefore will be 3 times messier
Turtles also need a UVB light in order for them to metabolize the calcium in their diet to maintain a healthy shell and bones. With this in mind, they also need calcium in their diet, in addition to that found in their pellets and other food. You can buy a cuttlebone like those used for birds. You just need to remove the hard backing and place a piece of the cleaned cuttlebone in their water.
I know this seems like a lot to absorb but in honesty, once you get their setup done up properly, you'll be surprised at how little work they can be. Most people freak out when they think of cleaning a larger enclosure but truthfully, the more water, the less work. It doesn't need to be cleaned nearly as often as a small tank with little water and no filtration.
Also, on the basking area/beach.. you need to make sure that whatever they have to bask on is easy for them to get too.. and allows them to completely dry off, top and bottom. Make sure you don't have small gravel in their tank. You can read many sad stories on here of owners who turtles died because ingesting it and becoming impacted. Another downside to gravel is that all the waste and food it holds and how difficult it makes to clean the tank and keep it clean.
I'm not sure if I covered everything but if not, I'm sure someone will fill in the missing holes. I do realized you just got these turtles and it takes time to get everything up to par..
