No it's definitely a water turtle. Absolutely no doubt.
How big is he? A full grown male will have a shell about 5-7 inches long and will need at a absolute bare minimum a 40 gallon aquarium filled about two thirds full of water. They eat in the water, snatching their food from the water surface from below. He probably will not eat unless he's submerged in water. They spend a lot of time swimming in the water, but they also need a dry place to bask and sleep, so you need a floating dock like this one. They also need a heat light above the dock. Unlike other reptiles, they don't require a UV light, they can get they vitamins they need from their food, but they would probably like it if you did provide one. An ideal tank would be more towards the 100 gallon range. Also, you will need to change the water a lot. I had 2 full grown turtles and no filter could keep up with the mess they made, so I was changing the water every 3 days. With a siphon and a hose it's not that big of a deal. Tap water will irritate their eyes, so you need to by some water conditioner too (dirt cheap, removes the chlorine from tap water).
I assume you are in Norman, because you posted this to /r/normanok. The Petco in the university town center (24th and Robinson) has these guys for sale. You can go there and look at the setup they have and talk to people that actually know what they are talking about.
Very cool. I will warn you that this tank is long and heavy. It took two people to cary it into the house. Also water is heavy so this setup will weigh around a thousand pounds when filled with water. Our house is on a concrete slab so weight is not an issue for us.
My turtles are smaller than your slider so I have not tried to build my own basking area. I use large Zoo Med floating docs for my map and painted turtles.
These are what I use: https://www.amazon.com/Zoo-Med-Turtle-Gallon-Tanks/dp/B00178LI50
They also make an extra large floating doc for larger turtles but I have not tried it out: https://www.amazon.com/Zoo-Med-78098-Turtle-X-Large/dp/B01AYBRVN8
I do not have a sump on the tank. I just run the FX6 and a UV sterilizer. Also the FX6 and power strip barely fit in the center compartment of that stand.
I used to use a syphon kit that I bought from the pet store to drain the water but that was expensive and a bit slow so I bought a cheap pump on Amazon and purchased a pipe to hose adapter at the hardware store. I start up the pump and let it run for a few minutes and then detach the hose so I can get the hose into the hard to reach corners of the tank.
Pump I use (it has a lot of different adapters for various tubing but no hose adapter): https://www.amazon.com/gp/product/B0018X2XT4/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
The adapter I have looks similar to this one except the one I have is male hose thread to 3/4" female pipe adapter (you can find these adapters in the irrigation section at Lowes and Home Depot): http://www.homedepot.com/p/DIG-3-4-in-Female-Hose-Thread-x-3-4-in-Male-Pipe-Thread-Swivel-Adapter-50007/100186564
I clean out the tank about once a month and clean reusable media and swap out filter floss and polishing pads. Even after a month my water is still clear and my ammonia and nitrites are at 0 but my nitrates are in the 40-60ppm range.
Older pics (pump I use with the adapter is in the second to last image in the gallery): https://imgur.com/a/2c4d0
Even older pics: https://imgur.com/a/xOieq