Garth, just changing the speakers is a breeze. Cordless screwdriver in hand, you can swap all 4 or 6 speakers in 15 minutes. It'll take you longer to unpackage the new ones than it will to zip the old ones out, reconnect the wires to the new and put em in. If you put the Sony 1620's in, you go to a 2 way speaker that can handle more power and will fit in the hole and same hole locations. Other speakers will work too. If you go non-marine, make sure they use poly and no paper. Also best if the woofer cone is solid and the tweeter/midrange is mounted on the frame over the cone than coming out of the center of the cone. Will help keep water from the voice coil.
Adding an amp in a boat is easy. Run power and ground direct from the battery to the helm area (best place for the amp is up under the dash high and dry). Make sure and put a fuse in right at the battery. If you're just running 1 amp, 8ga. wire should be plenty. If more in the future or now, run 4 ga. to distribution blocks and then you can run 8ga. from those to your amps. Run RCA's from radio to amp. If your radio only has one set out, many 4 channel amps only need one set in and will output to all 4 speakers. Connect your speakers wires up. To confirm they are correct in phase, use a battery (9V, AA, it doesn't matter). Connect what you think is the positive speaker lead that will come out of the amp to the + on the battery and the other to the -. Cone will move out. Cool. Do this to all speakers and connect then to the + and - outputs at the amp. Tie everything up nice and tidy. Start with the amp gain below mid. Then, with a good music source (loud cd or something), turn the head unit up to about 28 and then start gaining the amps up, one at a time, one channel at a time till the speaker starts to distort. Back off slightly. Do the same for each. To isolate, you can unplug RCA's from the amp inputs. Mentally, think of 28 as your "drunk proof" number. No matter what, don't go above this volume unless you can hear the speakers and know they can handle more. Some of the old music might need to be pushed louder. You should be pretty close....
O'
"Little Mascara 3"
2003 266BR,496 MAG w/Bravo III