It seems like they used dragon boats. You can see the dragonheads at the end of the boats in these pictures. The Khmer Empire depicted the Champa navy in their bas-relief carving at the Bayon temple. I think they were the only enemies which were depicted which says something. They are also depicted using spears and armshield. They are the guys in the helmets.
https://commons.wikimedia.org/wiki/File:Angkor_Thom,_Bay%C3%B3n_36.jpg
https://commons.wikimedia.org/wiki/File:Angkor_Thom-Bayon-86-Champa-Heer-2007-gje.jpg
https://commons.wikimedia.org/wiki/File:Bas-reliefs_du_Bayon_(Angkor)_(6912578329).jpg_(6912578329).jpg)