you can use the high level inputs as suggested by totalimmortal. if you pulled the wires that where used for the amps from the boat, make sure they are long enough for the civic. take into consideration that the wire wont run a straight path from your battery under the hood to the rear of the vehicle where you mount your amp, so a power cable about 15-17 ft is recommended, ground you want as short as possible and it needs to be the same size or larger in diameter than the power wire. remote wire also about 15-17 ft in lenght. what kind of radio was it hooked up in the boat? if it was aftermarket with 3 pre-outs, I would suggest using that in the civic. All that would be needed in a dashkit (10-20 bucks) and a civic harness (also 10-20 bucks). if you use the high-level inputs on both amps, you will be pulling the signal from the 4 outputs (stock speakers) to 6 inputs (4 channels for the alpine amp and 2 channels for the rockford fosgate amp), which means you will be spliting the signal and a high-level signal is not the cleanest route to take in the first place.

my suggestion is to have some patience...save alittle money and go out and shop for a cheap radio that has 3 pre-outs and buy a 4-channel rca and 2-channel rca, both about 15-17ft in length. you will then need the dash kit and harness. one other thing i would suggest is replacing your 6 year old stock speakers with some medium grade coaxles for about 60-80 bucks a pair. dont forget you will need wires for your amps which will run about 100 bucks or so.

all in all, you get what you pay for in car audio...if it sounds too good to be true, it usually is.