Well considering Applejack has been Apple Bloom’s mother figure, what with them not having a mother, it’s kind of understandable why she’d want to protect her sister. And she wore them in Leap of Faith. Maybe Apple Bloom can sorta swim but she’s just not very good at it. AJ’s taking precautions to ensure her beloved baby sister doesn’t drown.
 
I imagine this whole thing was Applejack’s idea and Rarity and RD went along with it.

I remeber when my mother did this when I needed those things, but could’nt blow them up myself. I feel akward now.

@Joshua  
“And now for your back leg floaties,hold still alrigth.”  
“Aw but sis I don’t need all this junk,you know I’m a good swimmer.  
“Remember all those us and Bigmac swam in the river,I was younger then and didn’t need any help.”  
“Yeah maybey but you was lighter then,now with all them apple fritters you been eatin you’ve pudged up.Can’t have you go belly up now.”  
“ I’m not pudgy,heck sis I went scuba diving with my friends a month ago and we were fine.”  
“Hush bloom we need to put on a second coat of susscreen,don’t want you looking all red and ugly like BigMac now do yah.”

Aaaand helmet.

Looks like AJ STILL hasn’t learned her lesson after “Somepony to watch over me”.

Pfffhahahahaha! So cute! XD