NEED HELP!!!! 86 2.1 volvo penta blown head gasket

[soccer082004]

Hey all, i have a 86 bayliner with a 2.1 volvo penta, 270outdrive. i just bought this boat from a guy with The head gasket blew and the head warpped. took head to machine shop and got it back to normal ready to put back together. I now need help figuring out why it blew up. i took the raw water pump off and apart, the impeller is in almost new condition and everything looks great. maybe gaskets, im not really sure. im not sure what he has done or what avenue to go to but i need to figure it out.

any help would be great!!!!