Those holes should be setup for 6 1/2 round speakers. Best bet is to look on the other side and see or cut the carpet and measure. As far as pre-wired, if it is, there should be some wiring up in the gunwhale on the starboard side of the boat. Other option would be to pull the radio and see how many wires are hanging off the plug. Should have a 12 volt constant, 12 volt switched, and ground. Then seperate from that there will be speaker wires. Most likely grey and white in color.