well if it's an ROV i think it's possible. if you want to do a perfect scale LA class boat at 15cm in length...it's basically gonna be the size of a pen. but if it's an ROV, it could be as large as a 15cm x 15cm box, and fitting components inside could be done easily.

honestly, i wouldn't worry about the material. i'd just find some sort of shape or container to fit everything together. maybe you could do what gulfstream suggested and use a PVC pipe about 15cm x 7 cm. then get some end caps. make some skids for the bottom of the ROV so it could sit on the lake/pond/pool floor. then, somehow, place the 6 motors in nacelles on the outside of the PVC pipe in whatever configuration you wanted. by the way, i assume this will dive by the propellers' thrust? or will this have a ballast system?