Doing some work on my dolphin boat I was using a tool I came up with to sculpt foam. It is very simple and cheap.
When I need to remove sections of foam from the inside of the hull when using the lost foam method this works great. It is basically a 100watt-soldering gun and some custom bent 12-gage wire. You can get these real cheap at places like Harbor Freight.
Just remove the stock iron tip and replace it with the 12-gage wire bent to whatever shape you need. It will take a minute or so for the wire to heat up enough. This beats trying to cut it out with a knife.
When I get it down to a smaller amount of foam left I melt the rest out with Acetone. In this boat I wanted to leave a good amount of foam in the front for emergency floatation. This tool made it easy to scoop out the front and leave what I wanted to save.