Complex sentence: There is no territorial sea boundary agreement between Indonesia and Malaysia for this portion of their maritime border, despite the southern portion possibly having overlapping claims because of the proximity of the the shores of the two countries as well as that of Singapore.
Split into simpler sentences: There is no territorial sea boundary agreement between Indonesia and Malaysia for this portion of their maritime border. Although no overlapping of territorial sea is apparent for most part of this segment of the border, the southern portion of should still see some overlapping claims because of the proximity of the the shores of the two countries as well as that of Singapore.