Who Makes A Good Floor Safe?

As already sort of noted this sounds like a really poor idea. If in a slab of concrete, laying horizontally, remember some important facts.
1.Water is heavier than air and settles on floors. Your safe better be the most water resistant one on earth.
2.Every time you open it, moisture would fall in and stay there. You would need some reeeeally fantastic gel packs for this system.
3.Concrete is really a good absorber of moisture. Therefore it is almost always damp especially the floors of houses, garages, etc.
4.Almost all safes have a locking system on the front. Makes for great tripping hazard.
5.Most safes are fairly deep. Just how deep of a concrete slab are you planning making? Might have to be several feet thick or formed for just that safe.

Now in a wall slab, might be a little different. So are you saying floor or wall slab?

French drains are usually used where only highly water proofed items are inside. The problem with them is under high water talble areas, water goes in as well as out. Exact reason not used in many areas for sump pits, handholes, manholes, etc.