How to translate this statement into Proposition variables

Question

We will go swimming only if it's sunny.

I was thinking this could also be written as : if it's sunny then we will go swimming

now I assumed that

P : we will go swimming

Q : if it's sunny

so finally I came to the conclusion that P <=> Q

but answer is P => Q only. how and why it is not biconditional ?

Comments

$P \rightarrow Q$ means$:P$ $only$ $if$ $Q$

see this from the book by Kenneth H. Rosen