location of amps in boat! any suggestion

If it's the same size that may be fine as long as the other existing boat circuits sharing that ground are not being used. If you are using the boat equipment that is on that factory ground cable and then use the stereo this cable will not be large enough and limit or effect how things work.

One thing I also found out on boats that pound in and out of the water is that any components inside of the amp that are not securely fastened down (like the big capacitors) will  flex back and forth until the leads on them break off (just like the old hanger trick, bend it back and forth enough and it will break apart). Take the amp cover of and see it you can wiggle any of the components around on the circuit board. If so, add some silicone to them on their insulated area's and to something else close by to lock them down. Let the glue dry a few days before powering the amp up (silicone when it's wet conducts electricity more readily).

Jim/Saint Louis

Running a ground cable to the fuse box would be the same as using the fuse box except you wont have to rely on the fuse box's ground cable. I don't think the heat in the engine compartment should hurt the subs(as long as they are ones like the marine Kicker subs)because they are made to be outside in the heat. If you are using two amps make sure to run the same gauge ground cable as your power cable(both should probobly be 4 gauge). So go ahead an put your amp or amps under the console and use a seperate ground cable. I think it should work fine. Post back if it dosn't and we can try to help you.

I have a mariah 222 open bow. Both amps are under the drivers console, out of the weather but exposed to plenty of air circulation. The ground is run to the console but it will work just fine if you run a wire to the batt.  Inside the hatch is not optimal for either amp or subs, and in reality, I just would not do it. my subs are under, and on either side of the rear seat which runs  the width of the boat. What used to be storage on either side of the seat has now become custom boxes. They fire up, are protected from sun and water, and modifications (just a very small amount of 1/2" plywood) were made to make sepearte enclosures for each sub.