The most common and most likely answer to this crossword clue is the 11 letter word BELLY-FLOPS
B
E
L
L
Y
-
F
L
O
P
S
Buddy explains!
The straight part
We know that "getting into the water tummy-first" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case getting into the water tummy-first = belly-flops.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BELLY-FLOPS mean?
BELLY-FLOPS noun- A style of diving into a body of water in which the surface impact is made mostly by one's abdomen.