How to install floor in empty sailboat

A bilge is where you will stow things and where the moisture from condensation, rain or waves will eventually collect. The gravity thing, drags it to the lowest point. This is where your pumps will live, so this moisture can be tossed over the side. All boats will get water in them. Leaks, spilled beer (a mortal sin), rain and splashes or boarding waves will get on the boat, guarantied. Small boats are much more prone to this then larger ones. Your boat is a small boat.

Tabbing is the method of attaching structural elements to the hull shell. It usually consists of strips of fabric, 4 to 6 inches wide, half on the hull and half on the piece being tabbed. This ties the piece to the hull, making a solid homogonous structure.

You can have a low sole, you just may have water above it from time to time, depending on angle of heel, how good you pumps are and how tight your boat is. A boat of that age will have leaks, probably lots of them. Stanchion bases, hull to deck joints, hardware attachments, thru hull fittings, etc. are common places for leaks. At one foot, you'd have a reasonably deep bilge for that boat, but check your headroom, you'll want 4' at least, to be comfortable sitting on a low settee or berth.

The sole can be removable, but I'd recommend the perimeter be bonded (tabbed) to the hull, with cutouts for the lift out sections of sole (called floor boards oddly enough). The removable sections can fall on a lip, mounted under the sole material. This is a common feature found in boats. Lots of access is good, don't be shy about cutting things up.

Very few sailboats have a level sole. It's designed to drain to the sump (the lowest point of the hull and where the pumps hang out). This should be obvious reasoning, but you don't want water pooling against things, like cabinets or other interior furniture, it will just promote rot and other problems.

With the boat kept in the water, you'll have a very difficult time finding the lowest point (to mount your pumps), let alone using a bubble level. Each place you step will alter the balance of the boat slightly, making using any kind of level frustrating. This is work, much easier to do on the hard, as I'm sure you can guess. Good Luck . . .