From the bottom up: Floor material (hypalon, glued to the tubes at bow and sides, transom at the back), keel tube (inflatable, I think it's anchored at the front and back, floating between those points), then floor.
The water level is likely changing due to all of it running back towards the transom when you accellerate.
To get it out, raise the nose of the boat and remove your drain plug(s). If possible, you can roll the boat over (hard to do with an engine mounted.) A third option would be to remove enough of the floor to get a wet/dry shop vacuum nozzle in and suck it out.
If the level is not changing, you like don't have a leak; rather it's rainwater that's collected or seawater that's sloshed in over the tubes (or possibly migrated through your drain holes - they never seem to be as one-way as they're designed to be.)