i have a 1991 5.7 mercruiser that has a 130 hours on it how do i pick a cam for it

see a marine specialist....reversion should be a major concern w/o dry exhaust. try a stock 350 mag cam, with regular thru-hulls, you should do fine, but it really depends on the whole combo of compression, cam, etc....stay close to stock for reliability....unless going stroker....a bit more room for "hopin it up"..

i was thing of going with the comp cam 'Xtreme Marine lift.480''/.489''
Duration: 270°/286°
RPM Range: 1600-5800

If you want to do it correctly call RMbuilder AKA Bob Madara of Marine Kinetics. It will not be cheap, do it right the first time. That is way low hours for a 91, I would be sceptical of the accrucy of the hour meter. Do a search on the forum you will find a lot of people with success.

got the boat from my friends mom and dad they got it new that why i know the hours