Most likely, the pack would be floating or at least be close to neutrally buoyant. A water bottle in water weighs nothing. Clothes in your pack trap a lot of air and floats for quite a while, until all that air is squeezed out and replaced by water, then they become neutral or slightly heavier than water. Heavy tools are obviously heavier than water. If you pack a lot of tools your pack will drag you down. If you don't, your pack will float, at least for a while.

That being said, there are lots of situations in water where a backpack on your back is the last thing you want. For one thing, it's hard to swim. And the pack will get entangled in all sort of things.