Can easily kill Alpha Rexes when they are near the shore line (like lava on the Centre). Approach the shore at a right angle and aggro the rex, then walk backwards into the water. The alpha won't attack you but will turn back to land. Nom it a few times then rinse and repeat. Killed 3 alphas in one visit to lava with 11k health and basic saddle.
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!
When in water, the spino can move without using stamina, this means that in a fight, if you run into a deep water source, you don't need to worry about stamina if you need to retreat. Has an oxygen bar so you can't dive forever sadly, very fast in water though.