They are skittish but not anywhere near faster than humans. They have low torpor and health, so tranq darts are recommended. Tamed like any carnivore, kibble is Archeopteryx. Can be found anywhere in and around the swamps.
Diplocaulus are constantly picked on by swamp carnivores, even the sluggish Dilos, add on the fact that they are so painfully slow and it results in it constantly kicking the bucket.
For taming either take some strong tames to kill all swamp carnivores or, and this is worth noting, use a pteranodon/Tapejara to hook it out the swamp for easy pen taming
If having trouble finding one in the swamps try bringing a parasaur with you and use its detection ability to see all nearby carnivores hiding in the water and walk up to them to check if they are a diplo. as long as you have some sarcos or barrys to clear out the piranhas you will find one in a couple of minutes easy.
Can ride but only underwater. In order to ride, hold whatever your ride button is and click the option that says ride. Level up in oxygen, health, and movement speed if using this for underwater dives. Really fast underwater, and can outspeed megalodons.
Fantastic utility Dino. You take oxygen from it passively while riding. The oxygen lasts a long time too; this combined with its speed means you can explore anywhere under water. That and it's a chitin machine ;)