YOU SAID:

Yesterday I went to the beach and while I was at the beach I saw a turtle

INTO JAPANESE

昨日、私は海に行き、海にいる間に亀を見ました

BACK INTO ENGLISH

Yesterday I went to the sea and saw a turtle while in the sea

INTO JAPANESE

昨日海に行ったら、海の中で亀を見ました

BACK INTO ENGLISH

When I went to the sea yesterday, I saw a turtle in the sea.

INTO JAPANESE

昨日海に行った時、海で亀を見ました。

BACK INTO ENGLISH

When I went to the sea yesterday, I saw a turtle in the sea.