YOU SAID:
I went to the sea shore with my cousin
INTO JAPANESE
いとこと一緒に海岸に行ってきました
BACK INTO ENGLISH
I went to the beach with my cousin
INTO JAPANESE
いとこと一緒にビーチに行ってきました
BACK INTO ENGLISH
I went to the beach with my cousin
That didn't even make that much sense in English.