On land, I go 100% run speed on the turtle without spurs, 104% with spurs
In water, I go 108% run speed with or without spurs, as the Sea Turtle increases swim speed.
I expect the other mount speedups (gloves, carrot, crop) will not affect the turtle's speed in water, and that a paladin's Crusader aura will not speed up the turtle's swimming speed (but will make the turtle faster on land compared to water).
I don't think swim speed speedups will affect it, as they tend not to stack. You DO go the faster swim speed if you drink a swim speed potion and then mount up (about 135%, with or without the turtle mount). It's just whatever gives you the single highest swim speed.
It's possible that spurs et al. are "working," but just like other swim speedups, the turtle mount is faster and they don't stack.
It's also interesting to note that my priest can go just as fast on water using levitate (with the glyph to remove the feather reagent) and any +8% movement speed enchant to boots; adding his 2-point Body and Soul talent as often as possible gives him an average speed of about 120%
