My boat is an 1979 16 x 48 alumaweld, it still has the original floor in it and the original blue paint on the outside. It has one soft spot up front. The old floor needs to go. I am going to spruce her up with a new floor and paint the outside gloss white.

Diamond plate is not advised. You'd be better off cutting the plate (3/16 5086 H116) to fit, masking the edges or around stanchions with blue tape, cutting the tape edges with a razor blade to make radiuses etc, grinding (4-1/2" with an 80 grit flapper) the main field of the floor plate and painting on a non skid like Matson or equivalent. Commercial guys don't use diamond plate. For the builder, diamond only comes in small sized sheets and when you cut it to fit its a pain to get the diamonds to come out nice. you end up grinding off half diamonds etc. The other big one is that aluminum, when it gets glycol or slippery crap on it is a skating rink in extra toughs. The big one is when you slip and go down on your knees on diamond plate, you'll never use it again. Oh, 3/16 decking is normal with a 1-1/2 by 1-1/2 x 1/8 wall sq tube stiffener on 16 inch centers. Skip welded 3-9 staggered to support the plate. I know its my opinion and lots of you have diamond plate all over and love it or whatever but I've got a lot of experience with this and I wouldn't have it in my boat.

Doing a diamond plate floor is just like doing a wood floor. Trace out old floor board on the back (smooth side) of diamond plate, grab a Skil saw with a sharp carbide blade and cut away. You will have to cut at about half the speed of wood but it cuts easy. Wear some kind of eye protection as aluminum shavings suck in the eyes (don't ask how I know this).

It's similar.
Clamp your work down...aluminum will accumulate on the blade and the work will want to "buck" occasionally.
To minimize that effect, use industrial stick wax.
Cheap, works well for tapping, too.
Comes in a tube (caulking tube size), just run your blade into it to coat the blade.

I would cut a touch big and finish with a belt sander where the cut is visible, such as in the front area.

Edit:
Yes, eye pro is a must. A face shield would be best.
Aluminum chips fly in a very nasty way.
They hurt when they hit, too.