It's possible to get every drop out with a wet/dry vacuum. You will be able to tell how much you removed and see it come back if there is a leak, and see how quickly it is coming back.
As for where the water is coming from, it could be from above the water line
like a window leak, mast boot leak, stantion or other deck mounted hardware leak.
Below the water line
could be from a thru hull, propeller
shaft packing, rudder shaft leaking, keel bolts leaking or maybe even a hairline crack in the hull.
Water tanks and related plumbing can also leak.
The first step is to get the water out of the bildge.
Next: sit back and have a cup of coffee, wait about 15 minutes and see if any water comes in. If no water comes:
Next, start the engine and just let the propeller
shaft turn and check to see if any water comes in, a couple of drips a minute is normal, for lubrication of the seal, any more and you'll need to tighten the packing nut or repack it.
Next, see if you can see the rudder shaft packing nut and check it for wetness, it might not leak until the hull is going up and down from going over waves. If it leaks, same deal as the propeller
shaft, tighten or repack.
Next, fill your water tanks to capacity and wait and see if any water accumulates in the bildge. If so, you'll need to inspect the tanks.
Next, pressurize the water system and see if any water accumulates.
Next, check for leaks above the water line
. You can wait until it rains or, you can use a hose. The nice thing about a hose is you can start low and at the stern and work your way forward and up. This way, it will be a little easier to locate exactly were the water is coming in. The most mysterious leak I ever chased was on a Catalina 25 I had. She kept puddling water in the section of floor between the head and the V berth. I looked for this leak all season one year. One evening the forecast was for rain. I sat and watched for the water to accumulate. With a very bright flashlight, I first determined whether the water was coming from port or starboard. Once it was ditermined to be coming from starboard, I followed the 1/16" wide trickle up the side to a piece of teak trim. I removed the trim and found a small hole that had been drilled by the factory for the rub rail but it was miss drilled so there was no fastener in it. When it rained, water ran behind the rub rail, thru the hull via the drilled hole, down the side of the cabin and finally, in the floor. The point is, you WILL find it, it's just a matter of how much effort. Best of luck!!!