Replace some of the floor?

I know that floor replacement is a regular topic so I feel that I need to keep it up to date.
After reading a post that I might be carrying some invisible passengers (wet foam); I decided to inspect and sure enough I have some unwanted passengers.
Now what to-do, I drilled some holes and found that the bow had water and the stern appeared not to have any, could this be true? If so do I only replace from the pole forward or what?
Now for the fun part I see some have used a wet saw, some a 4” cut off wheel could I use a skill saw?
I plan on using closed cell foam, but on top of the foam was plywood and it appears to need replacing. Could I use closed cell PVC plastic instead of marine plywood or any other exterior material like exterior sign material (we use this at my business in place of pressure treated plywood).
Should I only do the front half of the boat and save myself some work?

I really need the advice on this one, and the motor has been put in a holding pattern.

how far up into the bow should I go? Till I see daylight