@Evowizard25  
If I remember correctly, only the baby sea ponies have the floaters. Thing is, their mothers disappeared early in the series.
 
Anyway, sea ponies alternate between swimming like seahorses and swimming more like dolphins, and can use those fins like hands. The vertical posture seems to be a resting pose rather than how they swim, and their find give them better dexterity than the other pony types.

@Amethyst_Crystal  
I know. I know. I just wonder why they need them in the first place. I mean they evolved to swim in the sea. You don’t see seals wear floaties. So having to wear a floatation device that’s probably only a century or two old seems detrimental.
 
And if the young need to be kept in shallower water and are poor swimmers, then why aren’t they accompanied by the adults?

@Evowizard25  
The sea ponies in the two specials, which came before the movie and show, do not wear floating devices. These are actually baby sea ponies throughout the show.

The sea ponies wear floating devices?….Sure I’ll buy that.
 
Though it does raise my interest on the evolution of sea ponies and their relation to ponies themselves.  
Personally, I prefer a traditional mermare appearance, {More pony looking frontal half with finned forelegs and a dorsal fin like that of Dolphins.}
 
Of course that would be more for fast swimming, agile type.
 
The type here seems more of a slower design meant to cling to sea-weed and other such items to save itself from being dragged by the tide.
 
{Though why do they have manes instead of fins? Manes wouldn’t be beneficial to an aquatic mammal. I mean, dolphins and seals don’t have them.}