This is a well-known song in Japan. Here're some Japanese sites with midi:
Vieni Sul Mar [mandolin arrangement]
Umi ni Kitare [Japanese lyrics only]
Umi no Kitare [Clikc on "No. 3"]
Umi ni Kitare
Umi ni Kitare [musical box arrangement; from THIS SITE]
Umi ni Kitare [from HERE]