Very fast in the water, not so fast on land. It can even swim through the swamp, which makes it a deadly mount or opponent. The main reason i tamed one of these is that it is the only swimming mount that you can use a spyglass on. It also is pretty strong and has decent melee damage.
During my first 30 days of playing on my single player world, I was able to take 4 of these absolute beasts. We were UNSTOPPABLE. That was until an Alpha Megladon absolutely wrecked all four of them. Can I get an F for my boys
Playing solo, can't recommend the Sarcosuchus enough. Tamed a level 76, it's 117 now, and now I own the rivers. A mid level Sarco can handle megalodons, mid level spinos, rexes, and at least low level alpha raptors. My higher lever Sarco hasn't met her match yet. I find combat with them to be awkward at times but they've got great health, stam regens underwater, and once you're in position they deal good damage quickly. Couldn't ask for a better early game mount.
Discovered a glitch on console where the Sarco can cause small animals to disappear by standing on top of them. I discovered this first by trying to stop a diplocaulus from running away, then by tested it twice more on an archeopteryx and a dilo.