Great core training for a mermaid out of water

HAPPY BELATED BIRTHDAY TO ME!!!!! woot woot!  

mom asks what i want this year. i of course reply 'a mermaid tail'  

day 17 of 21 answers in 21ish days::::::::  

from me and mom::::::::::i just got my mermaid tail but don't have a big body of water 'til later today. are there some core strengthening exercises i can do from my backyard?