Hitachi seaside park by JR east pass

Hello there, can I use the JR east pass (Nagano Niigata area) going to Hitachi seaside park? Thank you.

You didn't specify which pass exactly. JR East has several:
http://www.jreast.co.jp/e/pass/index.html

But for the Tokyo Wide Pass, Nagano Niigata Area Pass, Tohoku Area Pass, and so on, as you can see by looking at the coverage maps, yes you can use it.
Stopping off in Mito to see the Kairakuen Garden along the way is nice too.

OK I see which one you mean. Not a problem.

Pass covered until Katsuta station(JR), near Mito,
but btw this station and park by bus and other railway companies are NOT covered.
See the both official sites again.
http://www.jreast.co.jp/e/eastpass_n/index.html
http://en.hitachikaihin.jp/access.html

Mito is capital city of Ibaraki prefecture,
http://english.ibarakiguide.jp/access.html
http://english.ibarakiguide.jp/leisure.html
"Kairaku-en" also good and very famous in Japan as top 3 as long history,
but don't know match or not, depends on flowers season.
https://en.wikipedia.org/wiki/Kairaku-en
https://en.wikipedia.org/wiki/Three_Great_Gardens_of_Japan
http://www.koen.pref.ibaraki.jp/foreign_language/en/index.html
http://www.koen.pref.ibaraki.jp/park/kairakuen01.html