I don't have too much experience with this other than my one boat project but having read a lot on this and other forums, marine carpet is the way to go. From what I've read/heard, going to HD or Lowes and getting indoor/outdoor carpet isn't a good idea. The reviews I've read almost all say that those carpets just don't last long and all that trouble to strip out the original carpet from your boat will just have to be repeated in a couple of years. I suppose there are other factors as well, like whether the boat is stored inside or covered, whether you do a lot of fishing in wet weather, etc. Maybe if kept dry the cheaper grades of carpet might last a little longer but not like marine carpet. Also, good marine carpet doesn't stain as easily from gas, oil or blood. When I did my boat I was just covering flat shapes that I installed in the boat after the carpet was attached. That made it very easy. I suppose when you have to strip out and lay carpet inside a boat rather than carpet flat shapes like I did it must seem like a big project. Having to cut and fold the carpet maybe makes the undercoating seem like an easier solution. Remember if you are standing on the undercoated deck and it is wet it will be slippery - a drag on rainy days getting in and out of the boat or just standing on it to fish. Also, the carpet can hide a lot of sins and if you haven't already stripped your boat you don't know what's under there. There are lots of places to get marine carpet - Cabela's and Gander Mountain carry it but I got mine from a local carpet dealer. I just googled "marine carpet in [my town]" and came up with a couple of local dealers. Marine carpet comes in several grades also and I guess depending on how long you want it to last and how well it works and how much dough you want to spend, there are lots of choices.
