It's strong, very strong, but that's all. People here lie. It's like the slowest animal of similar size, unless you put it into running mode, slow in water equally as on land. Stamina does not regenerate faster in the water. Gets stuck everywhere. It's fast when you set it to follow, so ride an allo and set the spino on follow.
To be strictly honest the spinosaur is one of the best mounts in game if you are even wble to obtain one be sure to make it a high level one they are very squishy at first so go around killing small dinos before attacking big prey
TLC patch coming in March 2018 to add the ability to swap between 2 legged mode (faster attacks and ability to attack with claws) and 4 legged mode (Faster movement), as well as increased turning radius, movement speed, damage, and health regen while in water and for a short time after leaving it. Like so people can see!